I don't know much about cats but have seen plenty of litters born on a farm I was around when I was a kid and they seemed to cope fine without any human interference whatsoever, we had one cat come every time it got pregnant, had the kittens (feral) and left.
I would say that mum knows what she is doing, after all in the wild she would have to leave for more than 1/2 hour to catch food etc. I would let her settle of her own accord and tend not to interfere too much.
If everyone is eating and suckling OK then I would think that everything is fine.
Most of the cats I see sunbathing seem to love the heat, my sisters old cat has been spending the last week of hot weather in her greenhouse of all places, even when she carries him out he goes back in after testing all the other places round the garden! And he is black!!
Is this her first litter? I am sure you are far more worried than mum is, she is probably cursing you for putting her back in with the kittens!
Seriously I would try not to worry, I am sure that if anything was seriously wrong you would know about it, try and enjoy the experience.