50% Rh on seedlings

agree with others, it will be fine but if you want an boost use a cup or something to create a humidy well of sorts.

It may not be perfect, but it will be ok. I wouldn’t go overboard trying to correct it. It should grow fast enough anyway as long as everything else is correct.

cut a 2L bottle to make a DIY humidity dome, if you want... cut some holes in it for air exchange. Humidifiers are cheap. the evaporative ones don't leave a residue on surfaces, but with those you have to buy a new wick. (evaporative = some sort of wick or filter with air pushed through it ... so mostly just a fan with a reservoir.)

Depends on temperature. Temp and RH% should mesh.. you can't properly consider one without the other. Refer to a VPD table for stage of life recommendations. VPD is calculated with leaf temp, so subtract 3-4F off any atmospheric reading before referencing VPD table. The most important thing in this regard is avoiding extremely high or extremely low VPD. Rooting clones might fall into 'extremely low' category, i guess. So if you don't have tight control over your climate, it's not the end of the world.

It will be just fine. Cannabis has survived for Millenia without Princess Perfect conditions.

use a transplant bag or a cup

well its not perfect but it wont kill her normaly