Sort of. The Epic Level Handbook specifically states ECL in 1 or 2 places, and then references level and/or HD in others.

There are, so far as I'm aware, two ways to qualify for the [Epic] tag on epic feats:

1. Having 21 HD or more (being a level 21 character, you would have 21 or more HD)
2. Being a True Dragon of at least Old age (according to Draconomicon, the fact all published True Dragons already have more than 21HD by that age doesn't seem to matter).

The only other way you can get an Epic feat is by having at least 11 levels of Ranger, and that only lets you nab Uncanny Accuracy, not any other feat.

It's ECL 21... so +LA will count for it.

False Theurgy skill trick, "... you can adjust the spell’s verbal and somatic components to mimic those of another spell of your choice of the same level."

It does not specify "another spell you know." Does it have to be?

Not sure, but I don't see how could you mimic something you don't know.

I forget if it was a feat or a spell called Ambient Music: It made bard song into just pleasant sounding background noise without being obviously magical. You might be able to adapt something from that.

I'd say no but require a Spellcraft check to mimic a spell not amongst those the caster 'knows'.
DC... 20+ spell level (based on "DC15+spell level: Identify a spell being cast. (You must see or hear the spell’s verbal or somatic components.)")

What are fun useful little tools for level 12 spys 3.5 e :P

Googles of Arcane Sight are a must. Also Dust of Appearance and Dust of Disapperance can be useful. Elixir of Love can help you charm that guard that finds you. and so much more. Arms and Equipment guide can give you some ideas.

FFG 40k Query;
Do items/powers that say "ignore the effects of fatigue" mean you can never faint due to fatigue during it's duration?