VET!
Really? Our vet would be quite bemused to get a call about this sort of thing.
We had dreadful problems with shelly hoof when we first started with sheep, as per my old threads which you linked to. These were actually quite interesting to look back on - we certainly don't see anything near that bad these days.
The thing is, if you don't trim the pocket all the way up, it remains a pocket, so it continues to fill with dirt and the hoof continues to detach as it grows back, and you can't get rid of the pocket. If you like, you could try partially trimming (just enough to allow you to clean out the pocket properly, so you can pack with hoof putty), but TBH I didn't fiund this to be very helpful when I did it, and I would now trim out the whole pocket back to 'sound' attached hoof.
I also don't think that doing this will necessitate keeping them inside, especially not in the summer. I expect your boy is most likely lame because he has a stone stuck in the pocket that is causing him pain. If you can trim a bit and then clean everything out, he should walk normally again, unless there is some other underlying problem?
I've said this before on other threads, but the thing we changed was to put "Zinc Rich" Rockies salt licks in every field. I'm convinced that this has resulted in our sheep having much stronger hooves, and has reduced the incidence of shelly hoof and cracking etc. If you protect the blocks well from the rain (very important), they will last a long time, so work out reasonably cheap in the long run.