Having never played a character like that, I can't really offer improvements, but hopefully I can answer your other questions...
Unsure about feats, but "practiced spellcaster assasin" is a must, with song of arcane power you should just about get +2 (since maxed its +4 and you get -2 due to bard) on caster level. Does this work like that? Do the sublime chord levels stack with my Assasin caster level?
If I understood the scripts correctly, then sublime chord casting uses your primary arcane class level rather than its own. In addition, your sublime chord levels get added to your primary caster class. In other words, your sublime chord level is added to your assassin caster level, and spells you gain through the sublime chord class will also use that caster level.
I'm not sure where the -2 due to bard is coming from (though it is probably in Bioware's scripts rather than in the PRC scripts somewhere, so I can't find it). Practiced spellcaster and song of arcane power should stack with each other.
Just how exactly do i learn epic spells?
I've only done it once or twice and not for a while, but you need the "seed" books to learn the spells. You can find both the spell books and seed books in the PRC options dialogue (you should find that you have the feat for this in your class feat menu). Then all you really have to do is use the unique powers on the books to learn them, which costs xp and gold, then you can choose which spells to ready through the PRC options dialogue again.
How do spell sneak attacks work? I did try Beguiler/Assasin/Arcane trickster, but the lack of attack spells makes it kinda hard to test anything out
The only class which has a spell sneak attack that affects damage to my knowledge is the Thrall of Graz'zt. The beguiler's stuff only alters your DC and chance to overcome SR. This should work when your target is denied their dex bonus to AC. That is defined if any one of the following is true:
- Creature is paralyzed
- Creature is asleep
- Creature is petrified
- Creature is "cutscene" paralyzed (it's a variant of paralysis used in some spells like otiluke's resilient sphere as it bypasses normal immunity)
- Creature is blinded
- Creature is stunned
- Creature is entangled
- Creature is frightened (the fear effect that makes you run away from everything)
- Creature is dazed
- The creature has never attempted an attack or a targeted spell in the last combat session
- The creature is not considered to be in combat (the resting icon lights up orange when you can't rest due to being in combat)
- Knocked down through use of the Knockdown or Improved Knockdown feats (whether other effects cause them to be flatfooted is questionable)
- The attacker is invisible and the defender does not have the "see invisibility" or "true seeing" effects on (these are caused by spells of the same name as well as other abilities)
- The creature has not seen its attacker for whatever reason (seeing is defined by Bioware's function)
There's some more stuff relating to uncanny dodge, but that doesn't apply to spell sneak attacking stuff.