Micaiah neutral or def+/hp-

Yes, Neutral is better because in the event that she has to tank a magic hit, neutral has 3 more HP over the -HP one.

She has low health id stick to neutral. Even if you try her as an enemy phase unit you lose in defenses with those ivs overall as your defense will never be THAT high

On a double DD build +def -HP is potentially advantageous with additional support to her def, but I think I agree with the others that neutral is preferred because her def isn't quite good enough so I do like the bit of extra HP when tanking on her res.

As other have said I'd go with neutral because of the extra hp. Helps tanking magic as other have said but it can also be used a tiny bit of extra healing for allies using Sacrifice. While situational Sacrifice is really solid, healing and the ability to reverse debuffs is quite nice!

I'd go with +def -hp.
you lower her magical bulk, which have almost no effect on her peformance, because she will take almost 0 damage from most mages anyway, but increase her chance to tank archers more.