Sorry -- I didn't have MoF with me when I looked up the SU versus SP abilities part, all I had was MM2; the Spellfire part was off the top of my head.
As for the SU abilities in other forms, when you're in another form, you still retain class abilities, right? If you do retain these abilities, then I'd say you retain Arcane Fire in alternate forms because it is a class ability, not a true supernatural ability due to race, unless of course someone can point out to me a place where you lose access to all supernatural abilities.