1NT opening vs. 5card major

Answer 1

If you're playing SAYC 5 card majors, it's not. 1NT denies a 5 card major. If you and your partner have a different convention that you both understand and are happy with, you could do it, but it would be unusual.

Now if you want to pretend one of your spades is a club and open it anyway (psychic) as you really hate your 5card to the 9 spade suit, that's up to you. But you can't fuss when your partner is annoyed about missing a 5-4 spade suit fit that would make 4S when you go down at 3NT ;)