Interesting scenario and topic here. However, I'm gonna disagree with your thinking that this could've greatly changed the wrestling landscape in regards to the NWA.
I'll start by answering your three questions.
1. What if Ric Flair hadn't appeared for the WWF at all?
Well it would be disappointing and the speculation is endless, but it also wouldn't have made much of a difference. With Flair not around as your top heel, you still had the likes of Ted Dibiase, Jake Roberts, the Undertaker to take over the top heel spot. Or they could've turned Sid heel earlier. Or even possibly bring Savage back as a heel. I liked how the WWF turned out with Flair as the top heel from late 1991 through 1992. Perhaps instead of the Roberts/Savage feud, they could've put Jake as Taker's backer/handler so to speak and have him feud with Hogan. A Roberts circa-1991/Hogan feud would've been gold. Or they could've repushed Dibiase as a top heel as he could always go.
Ultimately I think even without Flair the WWF would've still evolved to what it did. Bret Hart and Shawn Michaels would've rose to the top and the New Generation would've came forth with most of the mainstays of the '80s/early '90s WWF era moving on (ala Hogan, Warrior, Piper, Roberts, Rude, Perfect, Slaughter, Snuka, etc.).
2. What if Ric Flair had stayed with the WCW/NWA instead
I don't think much would've changed there either although a few things could've been different. One, the Dangerous Alliance might not have formed due to Flair being around to keep the Four Horsemen going. Although that stable might have formed anyway so there could be two top heel stables which would've been a first in that time period (as this would've been about 5-6 years before the "Gang Warfare" period in the WWF). Another difference was that Vader may not have rose to the top. It was largely the absence of Flair as the top heel that allowed the spot for Vader to become the #1 heel in WCW. So that might've been a big change. Other than though, I think WCW as it was in 1992 would've been about the same with Flair on board. Although it probably would've been Flair and Sting feuding for the title again in 1992 instead of Sting/Vader. Or perhaps Vader could've risen anyway and an alliance could've been formed with Flair to take out your Stings, Davey Boy Smiths, Lugers (who may not have went to the WWF with Flair still being around), Cactus Jacks, Ricky Steamboats, etc.
3. Would the NWA still be popular had Flair never came to the WWF with the NWA title?
This question has a simple answer, no it wouldn't have. Flair going to the WWF with the NWA belt I don't feel played much, if any role in the downfall of the NWA (downfall is the correct word as the NWA is still technically around today, just not nearly as prominent obviously). While it might have hurt the image a bit, WCW was already on it's way to separating itself from the NWA, and WCW was quickly rising to the top as the 2nd top promotion in America. By the early '90s the NWA was largely forgotten and not paid attention to by the wrestling press and wrestling fans. I would say even the independent promotions like the USWA, the UWF, and Smokey Mountain Wrestling received more press and coverage.
The NWA declined because it's era had simply passed. The NWA's formation in 1948 pretty much created the territory system. All of your promotions throughout the country from Florida Championship Wrestling, to World Class Championship Wrestilng, to Mid-Atlantic wrestling, to Georgia Championship Wrestilng, to Stampede Wrestling, were all apart of the NWA. Even the WWF started out in the NWA. The NWA dominated wrestling from 1948 to the early 1980's, because the NWA pretty much was the entire landscape of American pro wrestling. Other than the AWA and the WWWF, pretty much all wrestling in America was under the NWA banner. Hence it's dominance.
But then in the early 1980's when the WWF revolutionized wrestling, the AWA briefly became a more public well known promotion, and when many of the wrestling territories went out of business, the NWA lost it's power and prestige. By the mid '80s the majority of the famous territories had went out of business and it was really only Mid-South wrestling, Mid-Atlantic and Georgia Championship Wrestling that were keeping the NWA afloat. This eventually evolved into WCW, and by the late '80s you could almost say that WCW pretty much was the NWA as most of the notable territories of the NWA were gone. Some new ones started up throughout the early '90s and even into today, but because the WWF became such a dominant promotion and came to be recognized as the true benchmark and definitive pro wrestling promotion, and because WCW eventually became it's own promotion, the NWA had nothing left really to carry it.
The biggest promotions from 1990-1996 were the WWF, WCW/NWA, ECW/NWA, Smokey Mountain Wrestling, and the USWA.
The WWF was obviously it's own promotion. WCW was moving away and eventually did. Smokey Mountain Wrestling was it's own promotion. And the USWA was promoted and run by Jerry Lawler and Jerry Jarrett, I believe on their own.
ECW was a promising organization that was somewhat keeping the NWA afloat (not by much but a little). And then Shane Douglas's speech announcing the formation of the real ECW we came to know and love also really hurt the NWA, much worse than Flair's showing of the title on WWF Wrestling Challenge or Superstars. ECW in my view could've been the NWA's last real hope, but when they severed their ties with the NWA and went on to become the biggest cult wrestling promotion in American pro wrestling history, the NWA was pretty much lost for good (as far as being one of the top promotions in the country. TNA was another one that managed to bring some spotlight back to the NWA but they also became their own promotion a few years ago, so now the NWA is back to being mostly independent wrestling with very little name value (in promotions and wrestlers).
The NWA was so powerful because virtually all of the notable promotions and wrestlers in American wrestling over a 40 year period were associated with it. The NWA lost it's power when too many promotions had successful runs on their own.
All of the major wrestling promotions over the last 20 years were either their own promotions, or promotions that started out under the NWA banner only to go their own ways and become their own organizations.
The NWA will never again reach the heights it was at until a big amount of wrestling promotions become serious contenders to the throne of the biggest wrestling company in America, companies, mind you, that start out and stay under the NWA banner and stay associated with the NWA.
So Flair's parading around of the NWA title on WWF shows was certainly not what brought down the NWA. It was the decline of the territory system, the system that the NWA largely created, that brought the NWA down.
