I should have prefaced, a horse that is moderately under muscled but properly fit will not be expanding multiple tree sizes. From everything the OP said, her horse is under muscled, probably a lot due to her saddle fitting nightmare she has been dealing with, but is still rideable and just needs topline and fitness. Flocking and paneling will change to accommodate the new muscle but fitting such a horse with a saddle a few tree sizes too big because you expect him to “fill it out” is improper.
Of course, if a horse is fit in a saddle too narrow they will magically “expand” multiple tree sizes when suddenly fit with a tree that was proper for them. Like a horse that is ridden in a saddle too wide, putting an appropriately narrower tree on doesn’t mean the horse lost a lot of muscle in the 2 second saddle switch. If I get myself into a size 4 pants in the morning and put on a size 10 in the afternoon, I didn’t gain a ton of weight that one day, I just was incorrectly fitted at the beginning.
To be honest, in my experience with the hunting world, people are not changing the tree’s on their saddles throughout the year to accommodate fitness. I have never heard that, never seen that and none of my hunting friends do that. It would be ridiculous. Not only is it expensive but few saddle companies allow for such frequent changes to the gullet unless it’s like the dover brand easy change system.
If a horse isn’t mature, they are of course going to be changing but hopefully their ridden career isn’t started so early that there are massive changes happening to those structures that would require such a massive tree change.
I still stand by what I said, and unless there are extenuating circumstances like being chronically ridden in a tree much too narrow or much too wide, going from super skinny and zero muscling to being round and muscled up in a healthy manner, or going from obese back to a healthy weight, there, I would expect some change in tree size. But for the average unfit but healthy and a bit under muscled horse, tree size, especially when measured as unspecific as the N, M, W system, tree change should be minimal if at all when properly fitted. I am more familiar with the Stubben system for tree measuring, 27cm being narrow and working up to 32cm for the widest. Change in tree width, like from a 28 to a 29 is very specific and a more accurate, small adjustment.
I think we are saying a lot of the same thing, just in different ways. You mention a lot of extenuating circumstances like getting a horse out of an improperly fit tree and for horses coming off the track. The racing saddle trees are incredibly small and place all weight right behind the wither, it doesn’t extend out to the seat on most racing saddles. They get very little, if turnout at all, are fed high grain diets and usually are comparatively very young so of course when their entire world changes, their body will too. In regards to your hunting point, I do not have your experiences, I am pulling from my own and changing the trees out regularly absolutely isn’t a thing in my area. IMO no horse should have their condition drastically fluctuate to that extent. It is incredibly unhealthy to go from hunting fit (which is very fit for the first and second flights) to so unfit that the tree is vastly the wrong size and needs to be changed.