I guess the question is - are you proposing PTS or hoping to treat the pyometra? I'm not really clear from your posts. I'd also ask if doing the surgery that it is all done at the 24/7 hospital rather than a transfer post surgery. Hoping you have a resolution you are comfortable with .
What is your turnout like? Is it large, pasture with hollows/hills - or flat agricultural? Can you try him in a barn/large pasture arrangement? The fact he weaves and stresses when horses move suggests he isn't quite happy. Sometimes unless you have experienced horses living out in truely proper...
Another thought- build a shed for the kids to play in, with heater + kids stuff at current place? Then no car, they've a safe, warm,lit place to be and you have your own place which in 5 years time may feel more appealing! Could you also consider a trial month for kids pony at new yard?
Could you get anyone else in to your current set up for company/help?
Without the children - I'd stay at my own yard.
With the children i see the attraction of being somewhere with other kids - makes it much more fun for them. What age are kids are they anywhere near old enough to be left at...
If you would consider further work up I would ask to be referred to a hospital with an equine medical clinician as they will get you the best chance of a recovery . Low protein is not a great thing to have.
Your latest update also sounds like you are scared of him - this + an excited or unhappy 5 yr old has massive ramifications. I'd suggest see if you can weedle out some local suggestions of a good young rider ( cheaper than pro) who is good at riding youngsters to come ride 2 times to see how he...
Number of possibilities:
Not enough exercise - 16 hours stabled is a fair bit. Does he need more work?
Ulcers - stabling + recent surgery- high risk for ulcers
Other pain
I think you need a competent vet to check him for pain +- scope or trial ulcer treatment + check saddle.
None of which are...
I thnk I would sit down and have a chat - but maybe approach it as you've really enjoyed the loan and wanted to check in plans for the next 6 months/winter
What do you want to do- do you want to ride on your own?
If you said to her you feel too big on the pony - do you think she might stop you...
I think the difficult bit is- if they weren't your in-laws - where would you keep your horses?
I would go into this very carefully. Are you planning children/got children? You need to present them with a reason other than 'I don't want to pay you anymore' as that is hard for anyone to swallow...