Hello DoH, DoM and DoW here. Materia hurts our heads.

We're not so smart on our end, we hit things until they fall down.
Here is how I not smarted.

So I bought Vanya and thought melding it would be so clever and I could be better at making monsters fall down.
I put INT materia in it, ignored warning, got two points out of four.
I did it again, got zero points.
Realized INT materia doesn't really go in it even though theres no information whatsoever until you're literally doing it.
I try to talk to FC and various active crafters around me but they are either antisocial or you guys have some vow of silence.
Internet says Mutamix can remove bad materia choices.
He doesn't give me the option.

What do I do?

First of all there IS an information how much benefit you get from the materia you want to melt. When you click on an item and than on a materia you will see in the window how much you get and if you get less than the materia can give it will be even RED so not to difficult to see. About Mutamix yes he can remove materia, though you will lose all you wont keep them. If you cannot remove them i dunno. I was a crafter already, maybe you need to have a crafter at abt 25-30 or something who did the quest for it, but as i said i am not sure abt that.

You should be able to remove materia by selecting "Small talk" when talking to Mutamix. You MAY be required to actually have done the Materia related quests already by having DOH classes high enough, though. Unsure.

Also, all items have stat-caps based on their item level. Most HQ gear (like HQ Vanya) comes with all of their stats pretty much "capped", so a Vanya Healers robe won't be able to accept more than a point or two of MND since it's already so close to its "cap". In that case, you want to put stats on the gear that it doesn't natively have (like, Spell Speed or Crit) to maximize your stats.

It's a stupid system and it's one of the reasons why crafting is so painfully gimped atm.