As has been mentioned, part of the problem is that plans seem to be changing constantly. Over the years, I've read many reports alleging that Vince frequently goes back & forth on things, changes his mind on this, alters that, etc. during WrestleMania season. These reports also allege that it drives everyone absolutely crazy, especially WWE's creative team as they sometimes have to rewrite entire ideas at the drop of a hat, sometimes even while Raw is in progress. I read a report a few weeks back stating that aside from Sting vs. Triple H, nothing is set in stone and that's highly unusual for this time. By now, the top matches of the show have usually been decided on since well before the Royal Rumble but, based on what I've read, that doesn't seem to be the case this year as Vince's final booking decisions are even more erratic than usual.
IF Reigns wins Sunday and goes onto face Brock Lesnar, it's a main event that's pretty much doomed to be heckled and hijacked by fans. IF Daniel Bryan wins, some dirtsheet writers claim that it shows a lack of faith on WWE's part in Roman Reigns. IF it winds up being a triple threat match, then some are going to complain that Vince has shoehorned Bryan into the main event due to lack of faith in Reigns, being out of touch with modern fans and underestimating Bryan's standing with fans.
If Reigns wins, then there's just nothing Vince can do but grit his teeth and take the heckling at WrestleMania.
As far the notion of him giving the match to Bryan or making it a threeway dance showing a lack of faith in Reigns, personally, I don't see how he couldn't be lacking in faith when it comes to Roman Reigns. Reigns has been a singles wrestler for all of 7 months or so, he's still pretty green, there are doubts among fans, roster members & legends alike that he has the skill to call a match and he's not being accepted with open arms by a significant number of fans in the way Vince was hoping or expecting.
As for the rest of FastLane, I'm thinking there'll either be a couple of unadvertised matches thrown in on the card. We might see The Ascension taking on the Prime Time Players, we'll probably get something with Miz debasing Mizdow, we might wind up with another Ziggler vs. Rollins match. In all honesty though, I wouldn't mind if the show just had 4 or 5 matches in order to give the matches plenty of time and really let the wrestlers tear it up. After all, you can be pretty certain that Cena vs. Rusev & Bryan vs. Reigns are both gonna probably be around 22-25 minutes at least and I suspect that the Sting/Triple H confrontation will last at least 20 minutes. As a result, that should be at least an hour, maybe more, for the top three attractions for the show. I've little doubt that Ambrose vs. Barrett and The Usos vs. Cesaro & Kidd could be fantastic matches if given a lot of time and the wrestlers were told to really just cut loose a bit. The Divas Championship match could also be strong if it went for more than the usual 5 to 7 minutes.
As to the build of these matches, I have to disagree with the idea that Bryan vs. Reigns has just been tossed in all willy nilly. They've put a lot of focus on this match over the past several weeks and have featured both guys prominently. As for the tag title match, again, this isn't something that's been randomly tossed together as these two teams have been feuding for over the past 5 weeks or so. The build hasn't been spectacular, but it hasn't been lousy. The Divas Championship match has gotten the usual build for women's matches on the main roster: the Bella Twins acting like they're still a couple of stuck up, popular, snooty 16 year old hot girls in high school while Paige is the outcast. Cena vs. Rusev has gotten some strong build over the past few weeks and this might be one of the more anticipated mid-card title matches WWE's had in quite a while. The worst build for any match on the show, in my opinion, has been the IC title match. The build essentially has consisted of Ambrose campaigning for a match and "forcing" Barrett to "sign" a contract last night, which definitely seems like a Vince idea as there's no logic to it.