It seems that you may be debugging way too many things at once. Do you know much about the FAXBACK.COM internals?
Just for a test you know you can bypass it. FreeSWITCH will work just fine on a windows box, using the FAXModem stuff, and com0com driver.
That's what I use to have the M$ fax service talk to, and it works great. No hardware, and FS doing T38 right out of the Fax Service Server.
Also, are you routing the call to the txfax app?