RE: Purchasing over Goods In Transit
Transfer orders are not meant to track purchasing items, the are meant to track consignments, subcontracting items and other stuff. This means that, if you want to fully use transfer orders, you should manage warehouse at the vendor/s , not only your location / in-transit location. You cannot use transfer orders if you do not own items (of course if you're not preventing from being negative) .
E.g. if you create a purchase order, you should pre-load items to be purchased at the vendor location, then create the transfer order in order to move items from vendor to you central location. Otherwise you should have negative items in bins / locations (and i don't think you want negatives ;) ).
It could be interesting to add a few lines of code in table "Purchase Order" in order to create item ledger entries lines in vendor's location at posting time, and change Posted Whse. Shipment Lines source type in orde to make them invoiceable (pickable in pruchase invoice subform) while receiving transfer orders through whse receipts, but honestly i haven't tried it so far. I think it could be safely and in a few hours!
I hope you can understand the whole process