Stray Cat Mum Pregnant Again?

Four days ago we discovered a litter of six kittens living in our garage. They are healthy and doing well and quickly becoming socialized Mum is a stray neighbourhood cat who has been shifting her kittens around. She seems pretty settled now, however, I just learnt that she can get pregnant while nursing and her belly is visibly very large I have a couple of questions:

How can I tell if she is still nursing from time to time? How far along is she likely to be if it's this visible?

What should I do? Unfortunately we're moving overseas and can't keep mum or the little kitties permanently so I'm debating whether to take mum immediately to the humane society and find good homes for the kitties when they're 8 weeks - or should I take them all to the humane society? Or keep them and mum till they're old enough for new homes?

I'm a little weary of how being in a cage for two+ weeks at the humane society would affect the kitties development and suspect they may be better off with us where they have room to explore and lots of cuddles. But maybe I've just got too attached! Most importantly I don't want mum to get pregnant if she isn't already. As a stray she doesn't take to being locked in!

Thanks!
Foster mum

Most Humane Society shelters do euthanize so I would be leery of taking the family there. Is there a no kill shelter or rescue near you who would be able to take mom and kittens or let you foster them for a time?
Mom can be spayed once the kittens are 8 weeks old which would probably be a good idea.
I know she probably puts up a fuss, but it's better for her, you and her kittens if she stays inside. She could get pregnant if she isn't already, she could get attacked by an animal or get a disease which she could pass on to the kittens, she could get killed and leave you with bottle babies, she could run away and abandon her kittens, etc.
When do you move? The kittens can leave mom at 10-12 weeks. Maybe you could keep them until they could be adopted.
Do you have friends or family who would be willing and able to foster?
I know it's a big responsibility. The unfortunate thing is that it is kitten season and most shelters will be full to capacity. Kill shelters will have to euthanize the less adoptable pets. They do the best they can, but often they have to play a numbers game.

Best of luck to you and the family!

Devlyn