I doubt Batista would've turned down a Brock match 2nd from the top, not sure why they didn't try it out
I think he would have turned down being part of the second to last match, especially when it's for a match much bigger than the main event. Brock vs Batista is probably the biggest dream match that could ever happen for me, and with as big of names as those guys are no way it wouldn't main event over Undertaker vs Reigns. I don't think WWE could have ever gotten Batista for WM32 though, and I don't blame Batista for not wanting to work with WWE again after WMXXX and with his success outside of wrestling.
Undertaker vs Reigns doesn't make anywhere near as much sense as Triple H vs Reigns. Undertaker would be cheered over Roman no matter what the storyline would be, which would only hurt Roman Reigns. Triple H is hated by a lot of fans and Reigns will be cheered over him and if they really want Reigns to be a star, Reigns vs Triple H is the perfect main event with everyone being injured.
Lesnar vs Bray Wyatt doesn't sound good but Brock has faced everyone else that's healthy (Reigns, Triple H, Undertaker, he'll be facing Ambrose at Fastlane). The only match that would be somewhat fresh that makes sense is Lesnar vs Ambrose, but I feel like Wyatt has a better chance of beating Brock as he has the Wyatt family backing him. They also probably want Ambrose to be in an IC title match, so Bray is the better choice.
Undertaker vs Braun Strowman doesn't sound like a great match, but if we're talking about guys who can believably beat Undertaker, Braun is one of the few possible choices. This probably isn't Undertaker's last match, so we don't want to see him against Kane again yet. Ambrose wouldn't be a bad choice but again I think they want him in the IC title match. Whose really left? Ryback? Big Show?
There really isn't a great choice for this one and if WWE has faith in Braun being part of their future I can definitely understand why they chose him. He's a powerlifter, he's a member of the Wyatts who have a feud going with Undertaker, and Undertaker already faced Bray last year so the only other Wyatt that makes sense for him to face is Braun. Harper and Rowan are IC title level at best right now. Braun is still new so there's no telling where he'll end up. They might as well try to push him and see where it goes.
I do agree that the OP's Wrestlemania card looks better (mostly for Lesnar vs Batista), I just don't think they could have gotten Batista. If I was WWE I probably would of had Bray win the Rumble so he felt like a bigger star and just had Reigns vs Triple anyway since it probably didn't need the title, but I don't think WWE is to far off what I would of done with this very limited roster they have right now.