This is par for the course for what Americans tend to deal with, I would say the majority of sites/providers have had no RTP listed until the last year or two. It's gradually changing for the better but it's been really slow. It used to be hard to find RTP info on pretty much anything.
As you said, RTG/real time gaming is one of the providers that has variable payout settings on their games that can be chosen by a casino provider, so they will probably be one of the absolute last to provide this info. The casinos with the 88/91% settings aren't going to want their customers to know they are ripping them off (curious what casino extreme group would show.)
With RTG casinos you should probably assume that around 94% is about the best you're going to get. If you're not happy with that, then unfortunately RTG is not for you. I would guess there are exactly 0 RTG casinos on the high/97% model right now. Be careful with asking chat about RTP because who knows how likely it is you'll get accurate info, I tried asking some RTG casinos back in 2013-2016 what their settings were and found out years later practically every one flat out lied to me (including intertops and club world when they were actually respected.) At least slotocash/deckmedia had the honesty to simply not tell me instead of lying.
So yeah, unfortunately you just are not going to get high RTP's on an RTG casino. Best you can do is bad RTP's with 200% bonuses and lots of wagering rules. I got sick of all the BS that comes with these casinos and their bonuses so I cut back on playing RTG quite a bit, since I don't think the odds are very good if you're not using a bonus. This thread might tempt me to put in a short run at nova 7's though (with how I usually do at RTG games, the short run may not be just an expression of speech).
You have to remember that the max win based on line multiplier thing is pretty unique to RTG games online, and most people aren't thinking of that when they're discussing the issue. Usually you're right, changing lines (while keeping total bet the same) only changes the volatility unless the game has some kind of weird feature based on lines or win lines. But due to RTG's weird 50k max line thing, that's not true, as was very briefly brought up in post 18/19 of the thread you linked.
So as we said, if wins weren't capped, the RTP would be the same regardless of lines played with some rare exceptions. But wins are capped (reducing RTP), and playing fewer lines makes the cap hurt less, so presumably RTP would be higher. Maybe only very slightly, but for Nova 7's a lot of the RTP seems to be invested into max wins so it could be a significant question.
There was someone who worked at RTG who briefly commented on the issue but this was 12 years ago and not quite as many games around that might hit the cap:
Feature Guarantees - RTG -- How long do you spin? - Page 2 - Casinomeister Forum
From his post it sounds like on rain dance you just lost that RTP from playing max lines, meanwhile on triple twister they forced the player to play max lines and upgraded the paytable very slightly to compensate.
It's actually kind of weird that they don't force max lines on nova 7's. They could very easily compensate for it by changing feature odds though, but I'm not sure RTG has the competence to do that and they certainly don't have the transparency to tell anyone how lines affect the game/rtp.
