i think the devs need to add a ''lock the cursor on the game window'' option,until that you can try playing windowed but using Borderless Gaming,which put your game in fullscreen without borders.
i totally agree, although i've found some nice arrangements on musescore, i think your best bet is use a sheet music composition site like flat or musescore (if you can afford the subscription price) and just transpose whatever part works best for you.
You have a few options since I think they’re focusing on getting this DLC out more than a Mac port.
1) Try WINE, which is similar to how Rosetta 2 works for translating the older Intel software to Mac M1 programs. There’s a few options, so I’d try looking at /r/winegaming if you want to learn more. It’s pretty involved and may work. /r/linuxgaming may also also help you, since Mac and Linux both use Unix.
1) Use CrossOver, which is a commercial supported version of WINE (a real-time translator software for Windows to PC), and is a hell of a lot easier to use than WINE usually is. You can get it here for $60. It's expensive, but probably works according to CrossOver’s maker’s own tests on it. Link
2) You can try getting it running through a virtual machine. My girlfriend plays RoR2 on Parallels, and it runs… okay. Playable and enjoyable, definitely not able to loop on rainstorm and can't go past 30 mins on monsoon.