Did you try to install it as an .img file without Mounting Persist partition first? This is a problem of twrp but usually only the unofficial builds expose this partition. Can you check another recovery option?
your Persist partition might got corrupted. You can extract it from official fastboot rom and then flash it in twrp (check before that if Persist partition is exposed in twrp). Flashing this partition may break your Widevine level 1.
Good luck, werty
Q. 1 You can always make an upgrade from beta to stable or vice versa but if you got stuck in a bootloop, apps are crashing etc. you'll have to make a clean flash.
Q. 2 Stable updates are released usually once on 2-3 months so as the security updates.
Miui 12 Stable version is released - https://sourceforge.net/projects/xiaomi-eu-multilang-miui-roms/files/xiaomi.eu/MIUI-STABLE-RELEASES/MIUIv12/xiaomi.eu_multi_MINote3_V184.108.40.206.PCHCNXM_v12-9.zip/download
vulkan is a part of the graphics drivers and can be updated separately. The rom already includes it and there's no need to flash it except if you want to update it.
Magisk gives you the power to install modules and modify system files with apps but some banking apps can detect it and refuse...
Unfortunately yes, we will reach end of official beta support soon so you may consider moving to custom roms like tarakbumba said. For now the most stable one is unofficial Los 17.1.
If you have questions don't hesitate to ask.