Is First and foremost grammatically correct?

Answer 1

"First", can be multiple things.

It can be an noun,

Ex: He was first in line

It can also be an adjective

Ex: He was the first man on the moon.

Finally, It can also be an adverb

Ex: First, we have to buy marshmallows, then we can make s'mores.

First can be a noun, adjective, and a adverb.

Answer 2

First in priority is correct. Priority is a much-misused word. It is something to have, or to be in, not something to be. Better speakers never say something is a priority, but rather that it has priority.

Answer 3

"First and foremost" is an adverb so the concept of grammar does not apply until you use it in a sentence.

Answer 4

The word 'first' is an adjective, a word that describes a noun as foremost in time or order.