Alt er love.
why isnt mulig in the plural form?
"Everything" (and "alt") is a singular collective noun. For example, in English, you would say "Everything is fine". You wouldn't say "Everything are fine".
I saw "allting" used for "everything" in another exercise. Can i use it here too, i.e. "Allting er mulig."?
Yes, that should be correct, but wouldn't be as common as 'alt'.