Created a login especially for this as I’ve read through it all and have a couple of comments:
How long was the holiday he had when you were 8 months pregnant? (Just curious)
Another poster asked if there were any cultural reasons as to how they’d try and excuse this (none would be justified btw)
if it was me I’d message the person who warned me to let them know what has happened since and to thank them. I’d probably also send some screenshots too so they can make up their own mind (and judge them harshly within the family too). Pre-empting any spurious claims by them.
You’re right that a divorce is the way to go. You’re not being emotional, hormonal, unforgiving etc. You’re avoiding future misery. Go with your gut - they’re obviously terrible people. There is no rationalising their behaviour so don’t even try.
You now need to think a few steps ahead, admittedly at the worst possible time but you can do it.
The solicitors appointment is a good idea. Some workplaces have employee assistance programmes - check if yours has.
Go through any and all paperwork your husband has at home to find evidence of income etc for later child maintenance / divorce. Do this while your FM is there, even though you’re fucking knackered and likely feel like you’ve been run over.
Think about bills and if now is the time to apply for Universal Credit to see you through maternity leave and beyond. Hopefully you’ve already lodged a child benefit claim in your name.
Much has been said about it being good you’re breastfeeding for custody reasons. It probably is but if you need to, don’t feel bad if you switch to combination or bottle feeding in time. If it’s going well though, great!
Good luck. You’ve done the right thing xx