Knowing very little about ThirdHome (literally everything I know is from your posts), it seems to me 3 to 9 keys for off season is not all that different than 6 to 9 for peak season. Why does the range at the top end not change from off season to peak season.
The way keys are allocated is by a base key amount, which for a 4BR Regal Vista is 3, multiplied by 1-4 based on the seasonality. For non-peak weeks, the multiplier is 1; peak weeks it is 2; and super peak weeks it is 3. There are some locations that get 4 (IIRC in Hawaii), but none of the ones I use with ThirdHome. You can also name your own price, which this person did, up to 50% higher than the standard rate. Because January at Massanutten is non-peak it would normally be 3 keys, but the owner set it at 4 keys - the down side of doing that is you don't get the keys unless and until someone books the week, which might be never.
Based on your experience would a Regal Vistas property be an efficient/good trader in ThirdHome?
My measure of trading power in ThirdHome is how much it costs me in maintenance fees per key. I don't recall what Regal Vista costs in maintenance fees as I don't own there, but if it were $1,500 (which seems reasonable compared to a 4 BR Woodstone), it would be $1,500 / 9 or $167 per key, assuming that it's a super peak week. That's really pretty decent and about what I get for my better weeks elsewhere. On the other hand, this week would ordinarily run at three times the cost or $500 per key, which isn't great - at 4 keys it would be better, of course - $375 per key, but would run the risk of not getting any.
ThirdHome does periodically reevaluate the keys assigned to a resort based on reviews by members and on how often it gets booked. I had been using a Sheraton Vistana Villages week with them, originally getting a 2 key base with a super peak multiplier resulting in 6 keys for maintenance fees ~$1200, IIRC, but they reduced the base to 1 key, which changed the cost per key from $200 to $400 so I'm not using that one with them any more. I could see that happening with Regal Vista because of the multiple other ways of getting there, though it's about the only one in ThirdHome right now. Bottom line is that if I owned a week 26 or 27 Regal Vista, I'd use it with ThirdHome but wouldn't count on continued viability.
The other way it could work out better would be if you owned there and also in one of the mini-systems like WorldMark that will allow you to exchange in external weeks. Doing that in WorldMark winds up adding steps to the process, but can give more flexibility in what you can deposit in ThirdHome in order to avoid the issues with changes in valuation.
My thinking to this point is that if I ever owned at Regal Vistas, I would 100% want my ownership to be enrolled in RCI Points (unless my plan was to always use or rent the week).
However, I think there is a pretty good chance that to trade in ThirdHome the ownership must NOT be enrolled in RCI Points. Massanutten will not verify a reservation to any kind of external party if it is enrolled in RCI Points (I tried this playing around with a Woodstone unity SFX, and it was a hard no.) Their policy is to always refer the party to RCI, and RCI will absolutely NOT do it and facilitate an owner trading somewhere else.
ThirdHome verifies reservation just by the confirmation email, not by contacting the resort. I have heard from other folks with units in RCI points that they've been able to deposit their weeks with ThirdHome. It wasn't at Massanutten, but at a different resort that also doesn't verify reservations made through RCI.