1- I'm not sure i'd want such an extention of the ST above the TT. There is a reason that seat posts have a min insertion mark. It's to make sure the post extends below the junction of the TT and ST. Have your friend do a test build first with the extended ST and test to destruction, then decide if it's going to hold up for you. This is one of the answers to #3. The pro has done this trial and rejects or is good with designs before you ride the finished product.

2- Sloping TTs are common these days. Some lug sets exist for this design. Lugs can be tweaked a little bit for further angle changing. Have your friend help you here. he is the one building the bike and has the motivation to choose parts that both work as a final product as well work for his skill set. Another reason to trust the pro more, he has the experience in knowing which parts work for the design.

3- In theory if the design is the same, the torch control and brazing the same, the mitering the same, the paint the same (go on here with other building aspects) then the finished product is the same. but the finished bike is more the this to some customers. The name on the frame means a lot to some. The ability to fall back on a warranty has a value. Whether there's insurance (or not) could come into play if something goes wrong and some one gets hurt (and this might not be the rider or decided by the rider).

Last point i'll make is that I would be very hesitant to build a bike that the customer was providing the parts for and trying to control the design strongly. As the builder I have a stake in the finished bike meeting many needs, the customer might not be taking all of them into consideration. As a builder my reputation might be worth more then the profit. (Oh yes, you mentioned that there would be no profit). I have been is your friends position and ended up giving back my customer/friend's deposit to keep the friendship. Andy.