Wow... I've only guesses, and you've such a wide variety of fruiting plants and trees I don't know that any one thing would apply. Plus - I'm not very good with diseases ... after a quarter century of doing this (at the same location) - if anything needed a lot of spay care - it's gone!
I wonder if that shade gave you an initial fungus problem? I'm not sure what kind of humidity you have over there?
There's the bark-scrape test; starting near the dead ends, scrape away (not whittle) the bark down to the wood. If you hit vivid green, thereâ€™s still some life; if the green is mixed with brown, it's likely in the process of dying. Since everything's in pots, there's no hurry (I suppose) to 'replace' them. If I have a dead tree this time of year I insist on replacing it ASAP so as not to lose any growing time. You could squeeze in another pot, or not.
I have an overall guess, and it's a real problem when growing in pots: death by freezing. Most of us don't consider freezing weather will penetrate from all 'sides' of a pot, whereas if they're in the ground it only freezes from the top down... Here in zone 6 I'll "plant" my potted plants in winter by grouping them together and piling plenty of soil over & around them for insulation. Some I've actually planted
the pot in the ground! Your losses seem too drastic for any one malady, other than possibly frozen roots..? I know you get a lot colder than we do!
...But maybe these ailments fit a profile familiar to someone else... if so, speak up! â€¦This was simply my first thought