for mold spots I just use tilex or other bathroom cleaner. (i've been told its bad for the seams/thread but I never noticed any long term issues, but probably because I haven't needed it much my ski stays indoors)
I have used the dollar store's totally awesome and I think its the same as LA awesome, the $1 squirt bottle, i've used it on pretty much everything except seats and it works great.
if its not some kind of surface stain you might be screwed, do you think its coming from underneath?