Nice pics!!! Wat camera are u using???
I would take the seedling out of the propagator now and just put it on the box next to the propagator, that looks close enough to me. Obviously keep the unsprouted one in the propagator.
Your seedling isn't that stretched at all in my opinion. I wouldn't worry about its future it looks fine to me.
I don't use moisture meters I just pick the pot up and if its light I water, thats something you will get the hang of with experience.
As for ph, at this stage I wouldn't add something as aggressive as ph up or down to such a small seedling. Besides its in soil which I imagine is ph balanced at 7, plus the soil will act as a buffer should your water be slightly off the desired ph.
What ph is your water anyway???
P.s. Im starting to fall in love with that grow cabinet, I wonder if Father Christmas will bring me one!