Yes I'm totally fine with you using the Storm sub-class for your Source PC.
Hooray!
Let's brainstorm an appropriate Wild Surge table; I'm down to develop sudden disruptions of weather, sudden lightning, and so forth with you 
Bear in mind these Wild Surges are NOT meant to be like the whimsical possibly beneficial surges of the PHB wild mage; they're meant to be dangerous things you (usually) want to avoid.
Yar, I get that, and that's great in my view. That said, they shouldn't be SO dangerous that you (the GM) feel like you don't want to risk destroying the game because of a single random roll.
Or that the other players have trouble justifying their involvement with what is essentially a human nuclear device. I'll get to that in more detail shortly.
You're right that the existing wild surge tables for the 2 Source sub-classes need a little revision. Weird is too powerful; I failed to realize it was a damage spell and confused it with a more illusion/charm type effect.
Well, it is that. But it's one that does quite a lot of damage, potentially.
To that though, even the Elder sorceror's 'force explosion' power does the same damage 3 times...which is a lot. Not AS bad, but still very dangerous. Consider too that THIS damage has no save to mitigate it; get away or get dead. At least it doesn't kill the Source (though leaves him helpless in the aftermath which could be as good as, depending on the situation). I think the force explosion could be salvaged more or less by allowing a Dexterity save for half damage; or by making it a single big boom rather than a continuous 3 round doomfest, and possibly by reducing the toll on the caster to Stunned until a Con save is made rather than just Unconscious for an unspecified duration of time. Stunned is still bad, but doesn't let someone walk up and slit your throat quite as easily.
That said, I don't see these Wild Surges as common. This isn't like "hey I am angry cause those bandits stole are horses, then holy crap I'm freaking out and tearing open a hole into another world!!" It takes really strong emotion to trigger these Wild Surges (see the Source Weakness class feature), so I'd guess frequency might be roughly once every level of adventure or two. Which is why I made them big story-driving things. Clearly, however, you're seeing these Wild Surges as occurring more frequently...what are you thinking?
I'm thinking there's no guidelines for how common they are. It's -entirely- up to the GM. You provide the GM with 3 situations that provoke you, and the GM decides when and how to weave those situations into the story. It could happen any time. As often as the GM wants. And the reality is that any 3 situations I provide for, say, Talashia will probably be pretty common events, like 'mages being persecuted by witless proles.' Because that's what she's gone through, and that's something that angers her. The description of this weakness doesn't really give much sense of how often it's intended to come into play, but selecting three hooks makes it feel to me like it will be if not 'common' than not 'uncommon' either.
That's kind of dodging the real issue though, which is...how commonplace should sending your friends to another world with no hope of return be? How often is slaughtering a village by accident acceptable? I totally get that you wanted these surges to be more than just 'randomly cast a spell hah hah it's funny' but some of these are pretty over the top.
Especially in how most of them are pretty vaguely phrased, have limited or no defense, and can inflict irrevocable changes not just to the Source, but to the PCs with the Source. Which makes playing a Source something that would be (once it happens a time or two) very unpopular around the table, I would think. And gaming is a social thing. What people think about the character I play IS important to me. The existing Wild Mage table, for all its frothiness, is already fairly controversial at one of my games...because of its potential to detonate Fireballs in our midst, or otherwise give allies a hard time. Fortunately, it's a giant table that is roughly equally stocked with useful effects as negative, so that controversy is muted.
But a 1 in 6 chance to just get zoinked to Hades if you muff a Str check? That would not go over well.
Cheating roll-troll. Nah, just kidding.
It was necessary. For the greater good.
1. Does Talashia have bad blood with her father? Does she even know where her family is? Does she care?
Yes. Bad blood. Yes. Pretend I said 'yes' another ten times or so here. She doesn't know where her dad lives now; she did go back to the old house...it took awhile since she was very young when she saw it last...but there's strangers living there now. She sort-of doesn't care where he is now, but if she found out by accident or something it would start to eat at her. She'd probably wind up looking him up in that case, eventually.
2. She was sold to a witcher, which means she could have a connection to [MENTION=93196]Axel[/MENTION]'s or [MENTION=2820]Fenris[/MENTION]' witcher PCs. Something for you guys to discuss? Maybe the older witcher though he was doing the troubled father a favor since he was at his wits end? Or maybe there was something more nefarious about the witcher's buying the child? Up to you!
That was a big motivation for me making that change, in fact, was giving some PC hooks. The specifics of the deal made are sufficiently nebulous and in the past, and outside of Talashia's knowledge that they could be adapted to suit any dastardly GM plot needed. My default assumption is that Daddy just didn't tell this Witcher that he was buying a Bag of Crazy. Which might explain why he was so quick to sell the house and get OUT of there after the transaction.
3. Whether she was apprenticed at the Conclave of Mages or the Lodge of Sorceresses would have a definite impact on her relations with spell caster PCs and the story:
Apprenticing with the Conclave implies that Talashia was earnestly being trained though she might have been more than the mages could handle. During this time, she may have run across [MENTION=15132]Steve Gorak[/MENTION]'s druid PC (or any other mage PCs) during the end of his studies in Ban Ard (the sorcerer school in Kaedwen). Since Kaedwen was occupied by Redania during the Third Northern War, and Redania is the seat of the Witch Hunters, it's possible Ban Ard (or more likely a smaller offshoot) was betrayed and the Witch Hunters took it down.
Sounds plausible! I've no idea if witchers have a better relationship with the Conclave or the Lodge, but I expect a big part of the deal was expedience. It's worth noting that Talashia won't be great shakes at Arcana (she's not stupid, but she ain't wizard-smart neither), but I find it quite likely that the majority of her training was just learning some modicum of control.
Apprenticing with the Lodge implies that Talashia had a more personal relationship with a particular sorceress, and possibly was being manipulated. I would suggest Margerita Laux-Antille who specializes in curses (well, the warding of curses), since she'll come into play in the first adventure. Since Aretuza is on the Isle of Thanedd and it's very unlikely that witch hunters would manage to take it down, it's more likely a smaller group of sorceresses was betrayed.
I do like plot connections, and NPC hooks. Sounds like the Conclave may give a PC connection, which is great, and the Lodge gives her an NPC connection...also great. Win win. I'm fine with either. Perhaps we should wait to see how other backgrounds take shape to see which suits the story better?
4. Who was this traitor that Talashia sicked a mob upon? What was their motive? How did Talashia manage to stir a mob without becoming a target of their frenzy herself being an "abomination of magic"?
The traitor who betrayed her wizard/sorceress buddies (and her) to the mob of witch hunters. Who that is, precisely, I am not certain of until I detail more about those events. My sense is that one (or possibly more than one) of the resident mages wanted something that the Lodge/Conclave chapter leadership was protecting. Forbidden knowledge or whatever. So they arranged for rumors to spread, secretly cultivated some mobby leaders, and orchestrated an attack on the compound...taking advantage of the chaos to make off with whatever it was they wanted.
Talashia managed to escape by virtue of Plot Device. Now, my original idea was that she employed similar tactics to get folks angry and suspicious at this traitor...
BUuuuuuuuuuuuuuuUUUUUUUUUUUUUUuuuuuuuuut...
...it might be more interesting if said traitor(s) has to this point escaped justice. Giving her one of those 'motivation' thingies I've been hearing so much about lately. Seems like all the cool kids have one. And it helps pave over the rough spot you mention next...
5. Lastly, what propelled Talashia from her "time where she lost her way" into the current adventure escorting Odvara, a fellow Source, to the Posada region of Dol Blathanna to deliver her to the elves? I'm imagining something like Talashia being guided by lightning storms to Odvara's hometown in Upper Aedirn where she ends up escorting her, though we'll need a bit more to go on than that...
If she thought Dol Blathanna might have information about this traitor, that'd be one compelling reason. If she knows an old witcher/druid buddy is involved, that'd be another. The elves also know a lot of old magic that she'd be very interested in a chance to study in hopes of expanding her mastery of the Power. For immortality. And, uh! also helping people and saving kittens!
...but mostly immortality.
