Hmm... I think this is one place I'm going to disagree. No matter how carefully someone plans, it's always possible that, as a player learns the ins and outs of the class, he'll realize that what he thought was a good choice is in fact a poor one. I figure if someone wants to trade out a mystery so badly that he's willing to lose a feat, he should have that option.
Heh, I believe my point came for two reasons: first, I always tried to plan a sorcerer as carefully as possible to avoid swapping and second and most important, I believe it was the expression “lose a feat” that made me disagree. If you change for “becomes unavailable until the mystery path is completed again” I will have no complains ;-)
Not by my intent. My intention was that, if the shadowcaster wants to apply "Meta" feats to his mysteries, the meta-shadow feats are the only way to go.
That said, it probably wouldn't be broken if the DM allowed the ones you list. I just think it futzes with the flavor.
I fully understand. But the point is: is there any game mechanic to prevent or approve the use of these feats? As much as I like the flavor, I do not want to let the fluffy aspect interfere in the game mechanics, as heartless as this may be

At the end of this post I put some thoughts regarding the Ability Focus feat, just for fun.
Oh, I dunno. I think black fire has some viable uses.
One of the big problems with Black Fire, I think, it’s the damage of 1d6 per 2 levels. I’d much prefer the mystery inflicting less damage (5d4) but with a quicker progression of 1d4/level. Comparing to the sor/wiz repertoire, the Hail of Stone spell from Complete Arcane/Spell Compendium inflicts 5d4 area on a smaller area for instantaneous duration, but allows no save. I’d prefer a 1d6/level for Black Fire but comparing to 2nd lvl evocation spells from Spell Compendium (which generally use 1d6/2 lv), I believe it’s safer to allocate a maximum of 5d4, considering the duration of the mystery. If we compare the scaling damage from both versions of Black Fire:
Lvl Dmg Average
3 - 1d6 3.5
4 - 2d6 7
5 - 2d6 7
6 - 3d6 10.5
7 - 3d6 10.5
8 - 4d6 14
9 - 4d6 14
10 - 5d6 17.5
Lvl Dmg Average
3 - 3d4 7.5
4 - 4d4 10
5 - 5d4 12.5
6 - 5d4 12.5
7 - 5d4 12.5
8 - 5d4 12.5
9 - 5d4 12.5
10 - 5d4 12.5
At 3rd level, a shadowcaster using the standard Black Fire will inflict 1d6 damage, negated by save, affecting three opponents at most. At 5th level, he may learn Killing Shadows that IMO is far more effective than Black Fire from now on. Now, a 3d4/4d4 Black Fire at levels 3-4 can be serviceable until level 5 aaaaand at level 6 (if you follow the sorcerer’s mechanic) he could exchange the now- surpassed Black Fire for another mystery like Piercing Shadows or Shadow Skin

.
Finally, I ask for a clarification regarding the reflex save of Black Fire: If you cast this mystery directly below a foe, the save is rolled on his turn, when the damage will be applied, right?
But you're partly right; the shadowcaster doesn't have a lot of offense at those levels. Then again, I'm not sure they're necessary. He has plenty of mysteries that serve other purposes--stealth, protection, etc.--to make himself useful.
I agree with the nature of more utilitarian mysteries (congress of shadows, thoughts of shadow, piercing sight), but I fear for the shadowcaster’s ofensive usefulness if he is the only arcanist in a group of four heroes. Another mystery path containing a 2nd lvl mystery whose ranged touch inflicts starting 3d6/4d6 damage could be a good thing
Thomas Percy, I also love Killing Shadows. Regarding the noctumancer, I like the PrC but I’m trying to stick with the pure shadowcaster as the wiz/sor levels can get in the way of the evaluation ;-). Just a question: you said about an empowered maximized killing shadow. But using both feats, each extending casting time to a full round action, wouldn’t extend Killing Shadows’ casting time to 1 full round?
*warning: rules rambling below*
Ability focus: I believe it can be applied to ONE (Su) mystery or (Sp) mystery or even to one (Spell) mystery that was targeted by the Favored Mystery Feat. Considering this feat applies to just one mystery, if we look a 20-level progression with a Charisma of 31 (+10), I summarized below the Save DC for a mystery (Su) or (Sp) for each mystery level.
1 - DC 23
2 - DC 24
3 - DC 25
4 - DC 26
5 - DC 27
6 - DC 28
7 - DC 29 (needs favored mystery feat)
8 - DC 30 (needs favored mystery feat)
9 - DC 31 (needs favored mystery feat)
Another primary spellcaster with the same ability score and with Greater Spell Focus Feat would achieve the same DC spending two feats, but its effect would affect far more spells. So far, so good. An interesting point appears when we stack Path Focus and Greater Path Focus. A shadowcaster who purchases these three feats add +4 to one mystery and +2 to up to other two mysteries of the same path. If he wants to apply these bonuses to Master Mysteries, he needs to buy Favored Mystery as well. The question is: does this DC stacking become too powerful? For mysteries 1-5 I’d say no, as their difficulty class will not surpass 31. Let’s consider the progression below:
13 - Mystery: Prison of Night (Ebon Walls Path)
14 - Mystery: Dark Soul (Heart and Soul Path)
15 - Mystery: Tomb of Night (Ebon Walls Path)
Feat: Path Focus: Ebon Walls
16 - Mystery: Dark Soul (Heart and Soul Path)
17 - Mystery: Consume Essence (Ebon Walls Path)
Bonus Feat: Favored Mystery (Consume Essence)
18 - Mystery: Shadow Surge (Heart and Soul Path)
Feat: Ability Focus (consume essence)
Bonus Feat: Greater Path Focus: Ebon Walls
19 - Mystery: Army of Shadow (Shadow Calling Path)
20 – Mystery: Shadow Time (Dark Metamorphosis Path)
So, in exchange for having a really restricted Master Mystery path until level 19 and for the expenditure of four feats, it’s possible to have a DC 33 Mystery. It’s not to say there aren’t any other drawbacks. Consume Essence is a touch spell (reach mystery can help) and it’s a death effect. If the Favored Mystery and Ability Focus were targeted at the Shadow Surge mystery, it’d have the inconvenient of being a mind-affecting effect. Alternatively, the shadowcaster could invest on the Ephemeral Storm mystery (which isn’t neither a mind-affecting effect nor a death effect and it spreads upon an area) if he desires to change the focus to the Breath of Twilight Path.
Now compare this DC with other spellcasters. A wizard with Greater Spell Focus (enchantment) and with two fey heritage feats from Complete Mage (one gives +1 to Save DC to enchantment spells) could achieve DC 32 to all 9th level enchantment spells with four feats. If he wanted to surpass that, he could purchase Quicken Spell and Fell Frighten (Libris Mortis ) and use a quickened magic missile that automatically induces shaken condition (-2 on saves) on viable targets while using Dominate Monster on the same round. Six feats to expend with effective DC 34 on 9th level enchantment spells.
Conclusion: Ability focus can be scary, but I got the impression other spellcasters also have their way to maximize spell DCs. My initial impression doesn´t disapprove it if the player want to burn feats and mysteries so badly. I would like another´s perpective to this, however
PS: Razz, could you please show the URL of this article regarding vestige creation? It really interests me.