asking Wataru why he's working with the Fangires, he's the king of the Fangires you moron and has ALWAYS been working with them except for like 10 minutes in the middle of the Kiva arc where he was usurped.
That's just pointless nitpicking. That question was just supposed to show his surprise at Wataru (seemingly randomly) attacking several (human) Riders with the Fangires, rather than working on the coexistence of Fangires and humans like he was supposed to.
Why was Mio's actor brought in if she doesn't advance the plot at all?
Huh? So a casting choice is a problem now? She advanced the plot by making that alliance with Apollo Geist, anyway.
Why build up Amaki as your apology for fucking up Hibiki horribly and THEN kill her with SAGA? SAGA, FOR CRYING OUT LOUD.
Saga is an upgrade level suit that matched Emperor without the Zanvat sword, so killing base level Riders isn't weird.
I agree that their deaths are disappointing... still, this story hasn't finished yet, the Rider War World will probably disappear (considering how it was stated that the fusion of worlds would destroy everything), so we might get some kind of reset.
This post won't even budge your opinion anyway, but your loss. So far, personally, I think this has been pretty good for a Heisei Kamen Rider finale, although only because most Heisei Rider series had mediocre or bad endings (The only really great ones were Kuuga's and Blade's, and the later had too much forced drama, so I can't even appreciate it).