I had a problem where I would spam arcane missiles on my mage, and it would restart the channeling, spending the mana. I ended up doing this:

/cast [channeling]; Arcane Missiles

That means that it just does nothing if I'm currently channeling, so I can now spam the 1 key (which also makes my priest /castrandom Smite, Holy Fire, Mind Blast ) without worrying that I'm restarting my arcane missiles ever femptosecond.

Maybe using a conditional "fallthrough" like that would work.

I've never really noticed a problem with regular casting on my /castrandoms... at least for me, if my toon is in the middle of casting, nothing happens.

I didn't really check your code taht closely - do you have any /target or /follow, or /stopcasting or /startattack stuff in the macro that could be overriding a cast?