Much of your frustration comes from the limitations of antiquated processing systems the airlines use to process tickets. UA has made many improvements. In the past, only a single FFC could be used for a ticket; now, multiple FFCs can be used. Same with ETCs. In the past, FFCs and ETCs were limited to UA metal only, now they can be used for all itins. There seems to be a deep limitation in the system of two sources of payments, one being a credit card and the other being a credit. Multiple credits of the same type are combined first and then applied as a single payment. But there is a fundamental DoT requirement that in the first 24 hours that a canceled ticket be refunded back to the original source of funding, so combining payment of different types of payments (FFC, ETC, TB,...) becomes a problem to uncombined.

Now you could say, just have one credit will all the same T&Cs. That becomes a problem as the TB is considered a gift card which requires 5-year expiration while ETC and FFCs have 1 year expiration, and extending all credits to 5 years creates a massive accounting issue for the airlines. I could see ETCs going into TB but FFCs the largest total credit item is restricted to the original traveler and removing that restriction would effectively open a large secondary gray market for selling tickets.

So, the restrictions are not there to frustrate the consumer but due to different reason for the credit and some limitations of the airlines payment processing systems.

The infrequent customer does not really face this issue, the use of TB suggests a customer of higher sophistication, especially if using TB to stockpile credit card airline credits.