Interesting thread.
I'm astonished that fish n chips from a chippy is seen as a nutritious meal 
I really am. Its definalty a treat here!
Op my personal feelings about how we do junk food in our house are not really relaxant to you. I'm a big believer if over all diet is good then one soldi day if junk won't matter.
But as pp have pointed out this is nothing to do with the junk food.
It's obviously the straw that broke your back. But you didn't say if you took Jon home.
As for Rowing over your son, maybe he just thinks wow she loves me this much that she has charged over to save me from this meal 😂😂.
Would anyone expect their dh to support them every single time over thier Mil.
Well it depends on all sorts of things!! But if your in a relationship and your Mil is entwined in your lives and you feel side lined and undermined, by both dh and Mil then you have to serious problem on your your hands.
Some people think that every son is able to be the diplomat and mediate equally and fairly between wife and mum and judge when either one is is the wrong and say so nicely.
The reality is often a domineering mum has raised a son who is afraid to say no. The wife trying to be polite then gets sucked into this drama and unless she stands up for herself... Will then become a sort of baby sitter whilst Mil runs the show through her emotionally battered son.
Even in a good situation where the son hasn't been raised by battle axe mum, people managing may not be his strong point.
Your dh sounds terrified if his dm running out after her panicking.
This is what would concern me more out of everything.
If op is being constantly undermined and side lined... Then yes her dh does have to support her!! They do have to present a united front.
Seeing a kind lady a few times a month or year would present a totally different way reaction because it won't affect ops life on a day to day basis. I imagine gp who see thier gc only rarely would get far more carte blanch on what they do.
Ultimately if ops dh never ever stands up for her or even agrees with her on details like this. And op feels this side lined then I'm not sure what life she has ahead of her.