Q1. Depends on how you play and a tiny bit on lag. I take it you keep spamming buttons until you see a cast bar. I do that a lot too, however in doing so you might press the same button a few times too often and the game may misinterpret this, thinking you want your next action to be the same button again. The MCH casts are only 1.5 seconds (base) as opposed to 2.5+ seconds on BLM, so your window to already tell the game which action to use next is really broad. This is also why you should really try to stop spamming and await the proc with MCH since you have about 1 second after proc to decide which action to use next, unlike BLM where you only see Firestarter procs when already casting the next action.

Q2. Pretty much what Aeliott said, avoiding to run out of TP is your priority. You don't want to keep recovering over a prolonged period. TP classes are generally more on the burst AE dps side of things, so you empty your TP a bit and then switch back to single target rotation, but keep everything up that doesn't cost you (Bishop in that case) as long as there are enough targets (usually 3+). As for multidotting, I'd only bother with high HP targets, if your targets don't live for the full dot duration you're better off sticking to single target rotation.