I have been following your posts and really sympathise with you. I feel that being a mum is stressful enough without all the other added stress.
I know your husband makes you feel like the bad person here but maybe one option is to play the "bad guy". Guys likes to "save face". My hubby too, so if he is forced to make a difficult decision, I always play the "bad guy" so he can just "blame" it all on me.
I strongly feel that you nor your husband should "invest" in that house. If your husband wants to be fillal, then maybe agree to give some of the profits earned from the shares to help the MONSTER subsidies the down payment or something but no more. And the money will not be considered as an investment but just a one time gift from the son to the mum.
And if the relatives or his mum gives him more pressure, just play the bad guy and ask your husband to blame it all on you. E.g. I cannot contribute anymore cos my wife say she will get a breakdown. Or my wife already invested a lot of money for my child's insurance and I have no spare cash. She went ahead without telling me so I also dont't know etc....
Anyway, you only see them once a year, if they dont't like you, so be it. You MUST persist this time round cos I feel it will be just money down the drain in the end.