Any MC alternative will becomr popular when MS removes or closes down MC JE, which is imminent because presumably without it people can publish patches that bypass protections as described in the linked thread
OpenRCT2 is a re-implementation of Roller Coaster Tycoon 2; a business simulation game in which you build theme parks. It's similar to TTD (which was a predecessor from the same developer), but with much less focus on logistics and more focus on artistic creativity. OpenMW is a re-implementation of Morrowind, a fantasy roll-playing game that, by modern standards, is a bit unforgiving to beginners and doesn't have much hand holding. All three are from the 90s and arguably nerdy.
On the other hand, it's my perception that minecraft has a mature and nerdy following too. Lots of kids play it as well, but there seems to be no shortage of tech workers who are into it. I think minecraft's redstone has more to offer a nerd than any of the three above games. And there is a mature modding scene for minecraft, making the game even more technical. I think/hope throwaway290 is right, that minetest will get much more attention from capable developers once Mojang strangles the life out of Java Edition.
You can, but you have to change it in the config file. Instead of resolving this limitation, they documented it on the wiki: https://wiki.minetest.net/Controls#PC
I don't understand how things can be this mediocre for so long; other open source implementations of popular proprietary games aren't like this. OpenTTD, OpenRCT2 and OpenMW are all more feature rich and polished than the originals, so I don't think it's a fundamental limitation of volunteer projects. I think maybe, Minetest deliberately keeps the edges rough to stay off Microsoft's turf.