INT maple leaf - useful for Warrior?

Is it true that if I equip that leaf before I level up, I will get more MP when I level?
Will it come instead of HP, or not?

You will get more MP is you have either 10 / 20 / 30 / etc int with your equips at the moment you level. This doesnt come instead of HP.

for each 10 INT u get 1MP, and sadly Nexon rounds down so having 89 Int will only get u 8 MP =[

Does it work opposite? Like a 10 STR for a mage giving more HP

Nope. STR does not affect hp for all classes.