Piloting as a perishable skill - what skills go first and how to self-assess and mitigate skill loss?
I have no hard data answers to your questions. Perhaps others will. I can, though, offer some personal observations.

My last two jobs were at 747 carriers. At the first we had two weeks on, two weeks off. At the second, we had 12 days off a month, which, with a little seniority, you could arrange to have as a block.

For me, the first thing to go was my instrument scan, and I used to joke that it took only 4 days of inactivity to lose that. The joking aside, I did notice a slight degradation the first day or two upon returning to work after time off. I should note that my habit was to hand fly the airplane to the first cruise level, and then to hand fly from the top of descent, so I perhaps would notice a bit of degradation more so than those who used the autopilot as soon as possible and until as late as possible.

I retired 15 years ago, and it had been 30 years since I had flown a light single engine airplane when, a few weeks ago, I decided to check out in a Cessna 172. After three sessions, the instructor said he was satisfied. I wasn't and we went out a four time, and he signed me off for my first-ever BFR.

In general, I did a little better than I had expected. In one area, landings, I did a worse. The problem was the sight picture I had in my head from 10 years of landing 747s. On the first landing, knowing that I'd want to flare high, I delayed rounding out until my gut way saying, "if you don't flare, you will crunch the airplane." When I started the flare, the instructor said, "No, you're way too high." I delayed again, then started again, and the instructor said I was still too high." I delayed again, then absolutely had to start again, at which time the instructor said something like, "Well, you're still high but go ahead." That landing was followed by three sessions of nothing but takeoffs and landings. Each session I improved, but, frankly, I have yet to get a really good landing. My next session will be solo and will be nothing but takeoffs and landings.

So, for me at least, I conclude that we don't forget how to fly, but we definite do lose the touch.

After I get the solo takeoff and landings session in, I'm going to go out with the instructor and shoot a couple of ILSs. I'm expecting my instrument scan to be terrible.