After I completed the TZ Watch School, I went thru a phase of skill building where I bought a bunch of 2846 powered Octo's. Did the COMPLETE teardown, "Pro"Ultrasonic Clean, One-Dip Balance, and lube and rebuild.
Out of curiosity, is yours a 17 jewel or 21 jewel?
To your questions and observations:
1.) Don't be fooled by it "ran for 58 hours". Unless you had "eyes on it" for the final 10 hours after you observed it @ 48 -- chances are its timekeeping went out the window, slowing greatly over its final hours/minutes. They don't run for 58 hours of perfect timekeeping and just STOP. So if you were sleeping, then checked it the next morning and came up with the 58 hours, that is not an accurate measure of reserve. Also, slow beat tend to run a bit longer.
2.) Zigmeister might be able to clarify my next speculation. From my 'subjective' experience, watches with less jewels, or too little lube tend to have less friction and start up easy, and may even have an extended reserve.
3.) Why convert the 2846 to a GMT, when the DG3804 is already a slow beat GMT movement with correct hand stack and good DW alignment? IF only for the reason it is "Swiss" it seems like a lot of extra work.
Overall sounds like you have a good movement.