The backlogs are one of the reasons given for changing the law, 6 more years to take enforcement action. I haven't seen anything on H&H about this and most articles are about change of use to residential so there'll be a lot of people who've been relying on the 4 year rule who are unaware.
I just had this with my Chihuahua, in his paw. He had a few days on ABs but the swelling increased, so he went back to the vet and had a GA so they could investigate more closely. They managed to get the pus out, didn't find any foreign bodies (said they usually get a grass seed or similar)...
These stories do make me think that if your horse has needs that must be met due to safety reasons, do put it in writing and get written acknowledgement. I don't know how tight that would be for any insurance claim but it couldn't hurt. They might even take your request more seriously.
I've searched to see if this has been discussed and can't find anything. The levelling up and regeneration act includes changing immunity from planning enforcement to 10 years for everything, so the 4 year rule would disappear. It needs the TCPA to be amended and there's no date for that...
You can usually get a stable conversion kit for any size mobile shelter. I think you'd struggle to get anything deeper than 12ft mobile as they're made to fit through gateways. On the other hand, many offer a bespoke service so no harm in asking. I use Cozee Homes in Kent so no delivery charge.
String test him. There are a couple of methods and you'll find both on line with pictures. One from ergot to elbow and one from middle of the knee to coronet.
If your decision to bring her in was due to weight and the need to provide more forage, your decision to keep her out should take that into account. Is there enough grass growth or can you put hay/haylage in the field?