2021
1: Specifically, it uses SA2's jump animation, and the jumpball for spindashing and bouncing. The jumpball looks slightly different depending on context, it's pretty much solid when bouncing. An option can enable the jump to look and sound like SA1's jump instead. Super Sonic uses SA2's jump anim and switches to SA1's depending on his speed.
2: GIF below!
3: Nope.
4: I think people could come from Sonic Adventure, play SRB2 for their first time, and instantly feel comfortable if they were using SA-Sonic. Everyone knows mastering SRB2's controls take ages, so I'm pretty confident in this.
Not in the planning, but it's possible. There's no grind rails in SRB2, so it'd be for niche mods and require alot of coding. If Metalwario DOES make sprites, I'm absolutely implementing them for as many uses as possible. Snowboards, skateboards, "minecart"-riding, you name it.
A whole team, hm? With just us two that sounds a bit of a stretch.
Me and Metalwario experimented with the physics alot. We even tried out a direct copy of Adventure's physics from the source code, and in SRB2's environment it felt awful. So...I made a bunch of systems for Metalwario to tweak the settings live mid-gameplay as he saw fit, and we've settled on what feels the best for gameplay.
Don't get me wrong, his moveset is directly from Adventure, we're purist as hell about that. But his physics and the quirks at which he does things are tweaked specifically for SRB2's engine and environments rather than copied directly from Adventure. So you can see this as more of a updated/remastered Sonic Adventure gameplay-wise.
Also, I've implemented Sonic's beautiful new waiting sprites when I woke up, here they are in-game!