Of course Roman Reigns vs. Brock Lesnar will happen at SummerSlam. There's no other reason they'd end their Greatest Royal Rumble match the way they did, especially since they had Kurt Angle openly state the referee made the wrong call and Roman should've been declared the winner but the ref's decision is final anyway. As for whether or not he'll win, I think he will but I'm not 100% certain because of WrestleMania, but the story they've been telling here seems to dictate that Roman Reigns pretty much has to win, though I'm admittedly not a fan of that narrative. Their plan to get Roman Reigns over as a face, or at least getting him a face pop when he wins the championship, by stretching out Lesnar's reign to the point where everybody is sick of it would be a fine one if it didn't just result in the fans turning against the product. And maybe not in the sense where they stop watching (certainly there are those who have, and the sizeable crowd leaving during Roman/Joe was at least a step in the right direction if the fans truly want make a stand against his push), but we all witnessed how the audience treated their WrestleMania match, and unless WWE is throwing in another wrestler into match or some other element, it wouldn't surprise me at all to see their SummerSlam encounter receive a similar reaction. I'd prefer a massive walk-out though rather than just "Boring" Chants and Beach Balls galore, because that way if Vince doesn't interpret them walking-out as a slight against top guy Roman, he might interpret it as a slight against PPV's being like 10 hour events.
Also on a tangential note, I do not agree with the idea that Roman Reigns turning heel will only cause the people booing him to start cheering and vice versa. Now, if a heel turn leads to them developing his character into a more interesting figure or if the [hopeful] shift in match structure eases the pain of his push, then certainly those booing him will start to cheer. But if that doesn't happen, and I think there's a good chance it won't, after all Roman's current character is really only a babyface via circumstances (he's placed into a narrative where he's presented as the good guy but his character doesn't need to make any drastic shifts in order to transition into heel), then I doubt he'll get cheered. Especially since as a heel he's competition would likely be guys like Braun Strowman, Seth Rollins, and Finn Balor, I really just can't imagine most crowds cheering for Roman over them. Those who boo Roman do it because for over three years he's been heavily pushed regardless of their vocal opinion on him, and how transparently obvious it is that WWE is trying to cement him as the top guy. From his WrestleMania victories over HHH & Taker, to his this seemingly never-ending feud with Lesnar which would probably be more palatable if it hadn't cut off Braun Strowman's chance for a Universal Championship while he's was arguably at his hottest. Like I said, I could see it working out like with the 50/50 split, especially if people just take that as a win and cheer for that reason but honestly, I feel a heel Roman Reign would mostly amount to a bad-ass heel character (like the other Samoan Joe) and he'd be heavily pushed while facing other really over face characters, which really wouldn't be all that different from what we're getting now, so I don't see why that would change. I guess my point here is that a heel turn is not a magic solution and not guaranteed to get flip reactions, they have to use the heel turn as catalyst to development or else the only thing you really flipped was Roman's opponents and tag partners. And even if it would just flip reactions, so what? If you can get the people complaining about Roman invested in his character successful heel turn, there's a good chance that when you turn him back face there's a likely chance they'll remain on board.
