A: The circuit breakers in postwar transformers were designed to protect the transformer, not the trains. Modern transformers are far more friendly when it comes to protecting the circuit boards within all new trains.
I have heard of some operators with postwar transformers who have put low-amperage fuses in their track circuits for just this reason, but the darned things tend to blow under what might be considered “normal use” with older postwar trains, particularly the dual-motored ones. You may have to experiment until you find a fuse amperage rating you can live with as a happy medium given that you operate a mix of new and old trains.
Scott’s Odds-n-Ends, specializing in toy train electrical products, sells circuit breakers designed for layouts. Four breakers are packaged in one casing, and you can choose from 10-amp, 15-amp, or 20-amp ratings.
It really shouldn’t matter on which side of the two-wire track circuit you install your circuit protection, so the ground would be fine. Perhaps it may be wise to break down and buy a couple of the new power sources that are more compatible with today’s circuit-board technology.