my advice would be to keep quiet about the messages.
If he's constantly giving her attention when he's home THAT is when you bring up the issue of him prioritising her instead of you.
you need to focus on becoming financially independent asap - continuing a part time, distance learning course is secondary to that for now.
tell him there needs to be a change in the family dynamic.
First you want clarity on family income and expenditure - so access to all bank accounts, savings,bills, expenditure etc.
he's not a single man anymore - he's chosen to be in a committed partnership and a parent-so ALL money is family money, not just 'his' because he earned it/bank of mum n dad gave it him.
you want access to your share of the family money that you earn via being a housekeeper, nanny, teacher and general dogsbody - all of which are jobs that would be paid a wage if they were outsourced.
you do not want him 'giving' you money on a 'needs' basis- you are not a child and not asking for 'pocket money'.
give him a list of essential household/kids expenditure.
what's leftover after bills have been paid is to be split between you.
if he won't agree then you stop homeschooling and get them registered into a mainstream school so you can focus on earning money.
check out rents/benefits/schools you would be entitled to in the uk.
you've still got 6 months to get through at least before relocating.
so if you get the ball rolling now, then come relocation time you can just go your own sweet way if things haven't changed for the better.
it goes without saying that you need to start using contraception again - having babies without using common sense is what's got you into this mess.
forget getting married just so you can divorce him for money - that isn't going to do your self respect or self esteem any favours.
the house is in joint names anyway so that's taken care of, and you can claim maintenance from him via cms if he dicks about with it.
look into how much notice you would need to give the tenants if you wanted to move back to your house.
what happens to the rental income/profits? cos 50% of that should be coming to you directly.
also check the legal status of moving dc back to uk against his wishes.