The leaving the nest is normal my cat does that - just like we mums have a sneaky break when are children are sleeping.

I am sure it is possible that her milk could dry up if she's not eating and more importantly drinking esp as she is so young - it shouldn't and you don't always here them suckling - or I don't with mine.
The best way to check is to weigh them daily - I check mine on the kitchen scales and they should be gaining approx 10g a day but I don't think there's real cause to worry unless she's loosing.

You may already but feed her as separately from you other cats as poss - I feel they need their own safe supply. They are prob feeling rather delicate - I was after birth!!

Don't know about the poo issue - my cats isn't runny.

Emily