I just love Charlie and I think you should not pre-judge what the behaviorist might say.
I think its that Charlie kniows what is out there rather than he can hear and smell things, he is just a born wanderer and now he is safe in the garden , I would not allow him to raom again.
Concerning the coroner virus, Ithink your vet is being a bit over the top and if another cat had a flu jab, it should be Ok. I believe the really worstcase scenario would be that a cat could catch FIP from a cat who is shedding the corona virus......please correct me someone if this wrong info.
However that really is a worse case scenario and all the bits of the puzzle would have to be in place. Its a most unlikely thing to happen.
My brother has a cat who also carries the corona virus and he had been a neighbourhood wanderer and player with all other cats in the neighbourhood before this was known. My bro went to saee all the other cat owners and known were worried and since then my brother took on a kitten and none of the other cats or the kitten have got ill.
Its one of those things where a little knowledge is maybe a bad thing and in most cases there is absolutely no worry.
Charlie maybe missing other animal company and could it also be that wildlife that used to be able to get into the garden cant anymore cos of the catproofing.
He is still a very young cat I believe, around 2yrs? and there sounds like there is a lot of kitten in him still. You have looked after him so well and its so sad that he is breaking your heart. Wait and see what the behaviourist suggests and then maybe something silly will actually sort things out. I do hope so