As far as I can tell, /merror off doesn't remove the error from the screen or the "bwonk" sound effect. It just removes it from the log. Which is useful to prevent your chat log being full of error messages, but doesn't entirely fix the problem.

For the quing, that's why I mainly use GCDs (Cures, DoTs) since the que isn't as much of an issue there. You can also just spam the button.

The Seraph/Consolation/Eos one is a bit trickier, but that's why I say testing it out. I've tried it and had no issue, though I do sometimes have to make a slight allowance for the delay/more precise timing than what I'm normally using to que up next abilities since you do have to wait for the animation lock of prior abilities to complete before it goes through.

I've seen people deal with this by repeating lines, for example:

/ac "cure ii"
/ac "cure ii"
/ac "cure ii"
/ac "cure ii"
/ac "cure ii"
/ac "cure ii"
/ac "cure ii"
/ac "cure"

....which creates a pseudo que since the macro tries over and over again and you're more likely to catch one of those. But I prefer just using more precise timing myself. Especially in the case of that macro where I'm only ever going to use Cure 2 in very deliberate situations anyway.