Flashing a modem in Odin won't damage the phone at all. Modems are completely separate from the kernel and ROM on this phone.
Try alpha 3, it is supposed to resolve some GPS issues. I didn't have any GPS issues with alpha 2 so I don't know if it will fix yours or not.
Also I flashed FD16 modem a few days ago and it is running great. If you need it pm me and I'll send you the tar
"Is life so dear, or peace so sweet, as to be purchased at the price of chains and slavery? Forbid it, Almighty God! I know not what course others may take; but as for me, give me liberty or give me death!"
-Patrick Henry, March 23, 1775