mine did the same EXACT things.
replace the battery before you do anything else.
I even tested my battery. and it said it was good.
but she wouldn't jump off and we had a battery charger on her all night.
seriously its your battery. and if you can get your money back on that starter i would.
our starters rarely go out. they make that whirring noise after the car cranks but all it needs is a little lubrication inside. theres a write up about the starter in the MkIV forums on the vortex.
let me know how it goes