Aha! We have an answer! It is not the tack, at all. (Although the saddle is definitely a little small for me, it is not causing the main problem.)
Here's what it turned out to be:
This horse has a soundness issue in his right rear leg - he's got arthritis and was out of commission for several months due to lameness + recovery from that. He's no longer lame, but his *is* still a little stiff in that leg, and he's getting a little off balance in his hindquarters, and the net effect of that is that he's pushing his barrel out to the left.
Which, since I didn't realize this and compensate for it, was pushing more of my weight into my right (thus pinching my right ankle and causing that foot to go numb) and off of my left (thus causing my foot to float and the stirrup to slip back).
It took about a half-hour of riding, stopping, riding, inspecting, etc. to figure this all out.
So I had a great learning opportunity, whereby I found out that I can use my seat and legs to cause the horse to shift his weight between his butt and legs. I'm not good enough yet to really "feel" his barrel shifting onto my leg, but I can DEFINITELY feel when this all happens because I feel *my* weight go onto my right hip and my left foot start to float in the stirrup. Now when that happens, I need to bend his head left and press on his side with my left let and make some room with my right leg, and he straightens his whole body out. Voila! No more stirrup slippage, no more numb foot!
I'm sure there is some kind of technical term for this process, and if so, I would love to know what it is. At the moment, I'm just happy that I learned something SUPER useful this morning (not just about helping fix his shape, but about the relationship between my hips and his hips).
The saddle, is, unfortunately throwing me out of balance a bit and causing me to carry my legs in the wrong place, which is making it difficult to efficiently use my legs to keep him going forward. Even more unfortunately is that the solution to this is to close up my hip angle and carry my weight forward (so my legs can go back), because this means I'm getting banged up a bit on the pommel. Worst yet is that we're pretty much stuck with this saddle for now. :( Ah well, it will be VERY helpful in prompting me to keep my descent on the rising trot nice and light!