Struggling Betta

I have put my 2-week-since-purchase Betta under a good deal of stress this weekend, completely revamping his tank to go completely artificial (I’m fed up of detritus worms).
Whilst in his “motel” tank yesterday he went nuts on a chunk of prawn that I had put in for the cherry shrimp to feed on. I’m thinking that he has overeaten.
He became very lethargic and I put him back into his revamped tank last night. This morning I thought he had died - I took the photos of him lying on the silk leaf to show you all as these mysterious white dots that have been plaguing the shrimp seemed to be on him, particularly on his left side - it was as if his skin was decaying on that one bit and the white dots had descended.
I scooped him into a food container and it became apparent that he was not dead so I took more photos of his left side - this time showing no white dots. I gave him a 3 minute methyl blue rinse before putting him back in his tank.
On the last photo, the white shimmery dot by the little fin near his gill has been there since I got him.
I think several things might be going on here. Last week I treated the tank with salt to get rid of a huge worm on a shrimp (Holtodrilus truncates, I’m fairly sure). I’ve also had white dots falling off my shrimp legs and my betta’s head in salt dips, so I’m not sure it’s ich. My Betta has probably eaten his own bodyweight in prawn. I also started malachite Green and formaldehyde treatment https://www.ntlabs.co.uk/browse-products/indoor/aquariums/anti-white-spot-fungus/
When they were in the motel tank and in the revamped tank to treat what I thought was ich because of the white spots.
I then water changed the meds out when I saw how lethargic my Betta was getting. I don’t think he could have been more tampered with if I had stamped on him. But those white dots look exactly like grains of sugar so I didn’t want to do nothing.
Anyway, could you look at the photos and tell me what you think? I’d be surprised if anything wormy or parasitey managed to survive the flubendazole and fenbendazole chemical clean-up of the tank and filter media (preserving the filter bacteria) but if something had been on the Betta or shrimp, then it could have survived. But hopefully he’s just over-eaten…again…

I've managed to get a good look at him, and he's very bloated.
Could be constipation from his prawn experience yesterday (I have no idea how much he actually ingested but he was playing with it despite it being a sizeable chunk) or I suppose it could be the start of dropsy.
Would you recommend this?
Or I could just do nothing...but he has been struggling now for 18 hours

Interesting update - I have just got my husband to remove what I thought was a very large (the size of a big pea) very dark brown shiny snail - not that it could have been a snail given the chemical treatments I’ve been giving my tank (whilst fish were in tge “motel”).
It was a poo. Lots more very dark poo is now following. Betta is still very lethargic and resting on a top-of-tank silk leaf, so he can look up and gulp air without doing anything.
I’ve also noticed a sore/wound on his eye (see pic) but I’ve no idea where that will have come from. No pine-coning that I can see.
I’ve had healthier looking fish die on me so I’m not holding out much hope, but fingers crossed.

…and here’s the pic. Note to self - attach as soon as you say you will…
The sore/wound is above his eye. Hopefully not on his eye?

Just read some of @Essjay old posts - no “-fix” treatments for anabantoids. Also - side note - why on earth are they so expensive? The only “active” ingredient is just an oil!
I’ll use salt again for his wound and constipation, and sustain the concentration for a few days unless someone has different advice? This time difference between our continents is no fun!

Most products intended for fish use are expensive. Look at the price difference between API Aquarium Salt and plain rock salt.

For now I would just do more water changes using just water conditioner. If that doesn't help after a few days we need to think again. All medications including salt are not actually good for fish, it's just that sometimes not using salt allows a condition to worsen and in this case it's the lesser of two evils.

Well, he hasn’t died yet. He’s still being incredibly lethargic, his breathing and swimming are laboured, but he hasn’t given up.
The poo hasn’t developed much beyond about an inch, but is extremely dark coloured - particularly if he gorged on a prawn yesterday? In one of the photos you can see it.
The close-up of his face shows the wound above his left eye, which doesn’t seem to be any worse. I did wonder if it was his colouring, but he doesn’t have it in early photos.
His water is clean and his pump is baffled.

Hi Everyone,
No change today. He is resting at the bottom of the tank and has been all day, often with his pectoral fins whirring. No interest in food.
Water parameters are fine and he’s having 2 x 40% water changes a day.
It’s just so sad - he should be back to normal now, surely? I’m wondering if the dose of malachite green (NT Labs anti white-spot and fungus) I put in the motel tank on Saturday afternoon has caused this. Maybe it wasn’t just his gorging on a prawn, as yesterday he expelled the hugest poo I’ve ever seen a fish produce.
At what point do I consider euthanising him? He has been like this since Saturday afternoon so he’s been suffering for over 48 hours.