Do you recall if the tractor you had was overcharging? I havent had time to pull the harness out or apart. But a good visual inspection and lots of wiggle testing have not revealed any issues such as a short. My thought is that a shorted wire would trip the fuse instantly or for instance when a certain manuver pinch'es or rubs the wire. This mower will run for several seconds and it appears that as the charging voltage increases it will trip the fuse and shut off. I am begining to think that possibly the new regulator is no good. Otherwise disconnecting the regulator should make no difference if the short is still there right??
I don't recall if the tractor was overcharging or not, sorry. This one did the same thing by running for a few seconds or up to 20 seconds and then the fuse blew. I know what you mean by looking in the charging circuit because of the higher charging voltage but this one was doing almost the exact same thing and I thought the same as you but it ended up being that wire I described in my earlier post. I sure don't want to send you on a wild goose chase but you may save yourself some time and money by physically inspecting every inch of that wire and all the wires in that part of the harness.
Another helpful tool I used on this case was a short finder made by Snap-On which has a circuit breaker in it and that made it so I didn't have to replace the fuse to test it every time. You can get just a 10 or 15 amp circuit breaker from Autozone to use for testing purposes and maybe save yourself some money in blown fuses.
Another thought would be to check the red wire from the fuse holder all the way back in the harness, too. If that wire is intermittently shorted to ground, it could cause the same problem.