Problem DiL

I am with you, Fleurpepper. It must be very worrying if you are watching somebody drinking gin all through the day, especially where there is a youngish child involved. The OP has said that her son is worried about his wife's drinking too. I think most of us might be tempted to say something in those circumstances.
Visiting somebody who lives so far away is very wearing so I understand fully while the OP may want to go longer but I also understand the stress it might be for the hosts. It seems that a fair compromise might be for the OP to rest somewhere for a few days first and then stay with the son for as long as her invite allows. Maybe days out with the grandaughter can give DIL time on her own away from her MIL which might relieve the strain too.

"Maybe days out with the grandaughter can give DIL time on her own away from her MIL which might relieve the strain too."

No, this wouldn't work if the parents do not need time away from their kids if it were different circumstances

If the parents need time away from the mil - mil can entertain herself

I can remember back in the 1960s when I went for month long holidays to stay with friends in Morocco. However they were out working during the day and I entertained myself. Also I would take off for another city for a week at a time using Casablanca as a base, so we were not in one another's pockets all the time. I have always been confident at finding my own way around in other countries. Im sure they would not have wanted me there for four weeks solid even though they did have one or two maids do do the housework.

A hotel or an air B&B would be a good idea for at least part of the time.

You're welcome for two weeks? That's amazing! I certainly wouldn't argue about it. Still, I do agree it's too long to stay with them, rather than nearby.

You sound very demanding
Is everything about you? Do you realise other people are involved and have actual feelings?

Exactly! It’s not unkind it’s truth. No one wants a houseguest that king, especially one who by their own admission has made errors in behavior. Yet ops son and dil
Are still letting her come visit for two weeks, seems quite generous. They could easily have said stay as long as you wanted the nearest hotels are x y and z.

You’re a guest in their home so if you are not welcome for more than 2 weeks then you can’t expect them to be available for you for more than 2 weeks. If you try to push their boundaries, it is likely they will tell you not to bother going at all. They are a nuclear family and their first priority will be each other.