Nice - you might update your opening post to include a "My Players Stay Out!" tag if any of them like ENWorld.
At Level 1, I'd stick with +0 to +2 battles: I find my players walked through +0's, but not without getting into some use of resources (Healing Surges or Dailies).
Pitting 4 L2's against an L4 Solo Controller was almost perfectly balanced: it came down to "next hit wins" between one PC and the Solo.
By the time you're hitting L10, your range should be
much wider; my party handled an L14 Solo with some HP shaved off to keep it from grinding too badly, so I think your L13 Controller v 5 L10's should work out well - by that point you should have a really good feeling for the groups optimization, magic items, etc, so I'd say tweak the battle by adjusting the rest of the encounter.
I suspect the DracoLich will be a bit much for the L11/L12 final boss battle - as others have noted, they may
win, but the constant missing will turn it into a real slow grind.
Don't be afraid to do level adjustment - using Monster Maker or Monster Builder, its fairly easy to go up or down with any creature. Practice early on, with non-Elite, non-Solo creatures just tossed into random battles, and you'll get a very good feel for it. I try to include one "Custom", "Named", or otherwise "Special" monster for each combat, rather than having the gang slog through yet another batch of MM routine orcs.
I've gotten a lot of mileage out of mashing things about; one of my players will never forget the L4 ghoul that played like an L6 bugbear strangler!
Based on what you learn about their optimization in the L10 Gateway battle, you'll have real good ideas how to balance a Solo-plus-friends battle against them.
The advice you're getting from the others, especially
Flipguarder and
Saagel, is spot on. The way to make a Solo feel dangerous is to toy with the action economy by giving it plenty of off-of-its-turn attack powers, and let it "raise the stakes" when Bloodied.
I'm particularly fond of a Rage Drake's ability to do more damage, and tend to apply it to all my Solo's. Depending on your party's build, the ability to shrug off Stun/Dominate can be crucial. Giving it additional actions, such as Minor attacks for limited damage, or Immediate * attacks can really help. Also, typically, save its two Action Points for "when bloodied" rather than burning through them early.
Ideally, you want the party to feel like they
had to throw everything they had at it to win.
If you've got the time, it can be well worth it to have your players throw you their character sheets and run the encounter as a test battle, just yourself playing both sides. Obviously any individual fight is subject to 1's, 20's, etc, that may turn the tide, but it can give you an idea of if the defenses are too high, too low, etc.