LineageOS will never let you pass validations without root. https://lineageos.org/Safetynet/Totally disagree. If you flash the rom right then no issues passing validations.
But the moment you install magisk, its all gone and even if doing a clean magisk uninstallation the rom can get bricked aka partial rooting persists to which the only solution is to flash stock rom and start from zero!
Depends on the ROM. Many ROMs pass validation by default, some don't.Safety net fails validation with an unlocked bootloader and custom recovery, forget about root. Even if you don't enable root, custom roms don't pass validation.