Can I use my properly wrapped and stored MMF that I have had since Halloween? Is it still good?

I seriously would not use MMF that is almost three months old, especially if it is a paying customer. The expectation of a customer (irregardless of paying or not) is that you use fresh ingredients - 3 month old fondant is not fresh in my books.

Taste it.
How does it taste to you? Would you eat it?
If it is only marshmallows, xxxsugar and water it should not be "bad", especially if it was properly stored. That is why I say taste it. It should taste like you just made it.
If you have ANY doubt. Toss it and make new. Ingreds for that are rather cheap.

Considering manufacture date to store shelf...I am sure store bought fondant sits just as long.

I've noticed that old MMF will get a slight sour taste and smell. I'm not sure what causes it. But personally I won't use it when it's more than two weeks old, unless it's been frozen.

You can use it to cover a cake board with though.

I will only use MMF to cover a cake that has been made no more than 1 week before, but i will use it for 4 weeks for decorations that really are going to be eatten like maybe a number or figure. But fresh fodant only to cover a cake. but realy i don't think it goes bad i do think the texture and quality does change with age.

I will use some of my left over colors a week or two after they are made for little stuff but for covering the cake I make fresh.

I will use older fondant (meaning one, two weeks MAX) on decorations as well but Halloween?? That's just down right gross!!

My kids (especially my 12 yr old son) love to play with my old fondant I am a WMI, so I sometimes give him the plain frosted cake after the first class and some old fondant to play with. Hasn't killed us yet, but I wouldn't use it for a paying customer.

Mff is made with not easily perishable ingredients. It would do no health harm.
I make mine on the standing mixer, never touch it with my hands. So no important germs would colonize it...
When you buy your marshmallows they could be a year old in the bag, and you'd still use them.
Bacteria will not grow on sugar.
I don't put shortening in my mmf, but it would still be OK because it's something that lasts almost for ever too. The same goes to corn syrup...
The only problem with mmf and time, is that the consistency of the mmf changes a bit.
I would use it for accents for sure, and to cover the cake I would use it if the consistency was OK.
When you buy already made fondant, how much does it lasts?

We have to understand about the longevity of the ingredients to answer that question.

And madras650, your cakes are amazing!!!