Safe to delete everything in the Windows temp folder?

Settings > System > Storage > Storage Sense will clear it for you on a schedule if you'd rather not do it by hand. There's also C:\Windows\Temp, which is a separate folder and needs admin to clear. Just don't do it while something is installing.

Yes. Win + R, type %temp%, select all and delete. Anything in use will throw a 'file in use' message, just tick do this for all and skip. Nothing breaks.