Obviously you are a Resident but is MOM? Make sure MOM has a valid Florida ID, DL, or Military ID with her or you won't be able to get that ticket! They will card her even if you are buying the ticket.
Yes, the season pass is valid then, as there are no black out dates in May, and it will save you about $20 over a 7 day expiring Magic Your Way ticket with the park hopper option. Plus you will be able to use it till it expires next year except black out days of course.
So yes if MOM lives in FLorida also this is the way to go. If she is not a resident, you can't do this even if you are paying for the ticket, She isn't a resident she will not be eligeable for the Florida seasonal Annual pass ticket.
It doesn't matter if you buy the ticket now and get a will call sheet or have a voucher mailed, or even if you buy the ticket at disney, she will still have to go to guest services to buy/activate any type or Florida resident ticket or AP ticket or any type and they will card her.