This is interesting regarding the use of soda ash
http://www.pburch.net/dyeing/FAQ/sodaash.shtml
Read this regarding retayne
http://www.pburch.net/dyeing/FAQ/fixative.shtml
Just an FYI you can purchase PFD fabric ( pre-pared for dying) The Kona white is PFD . Not sure about the other colors.
If you are treating your fabric prior to the printing with Retayne the value would be very little as retayne works on the dyes that are already in the fabric. If you wanted to treat after the printing I am not sure the inks used in Ink jet printers would hold up to the 140 degree water ( required for retayne) for 20 minutes( the recommended time for treating) .