Sorry for not replying more, I'm on holiday at the moment so don't check back here much.

Regarding button usage, you can use all the numpad numbers unless you use them for something else. Or use the click commands like I outlined in a previous post to still get the regular functionality of buttons like O, C etc (I haven't tested this myself).

Regarding castsequence, I haven't tried it myself, but seems like it should work fine so long as you don't have it reset on target. If you want that reset, I would try doing a big /assist line at the start instead of /target, and hopefully you'll keep the same target (mob this time, rather than targetting the player), and it won't reset each time you retarget the same guy.