Thank you von Zelda and Sailor Steve.
With regards to playing 'realistic' I've been doing what Sailor Steve does: use magnetic until BdU says to stop, then go back to magnetic when I get the 'go-ahead' message. (By the way, does anyone have the exact dates for those official notifications?)
With regards to the actual, real methodology, from the research I've been able to do and with my own, real-life experience with fixed and semi-fixed ammuntion, I think the following is probably accurate:
(credit to Paajtor; got it from his siggy)
The nose is probably fastened/removed with a wrench or spanner, permitting the pistol body to be inserted and removed. It's probably the same pistol for both contact and magnetic detonation; you just disable or enable magnetic detonating. The fuze is part of the pistol (and possibly, but it doesn't look so, the booster charge). So in order to activate or deactivate the magnetic portion you'd have to physically unscrew the nose and remove the pistol; definitely not do-able once the torpedo is in its tube, and not something that looks quick and easy to do when it's
not in its tube.
I knew the gyro angles had to be able to be updated with the torpedo in its tube, and I was reasonably sure so did running depth. Thanks, Sailor Steve, both for confirming that as well as mentioning just how that was done.
As an aside, I discovered two interesting torpedo tidbits (both from wiki):
1) The [G7a] torpedo was of a straight running unguided design, stabilized by a gyroscope. The G7a was of variable speed, running a distance of 6km at 44kts, 8km at 40kts, and 14km at 30kts. The 44kts setting was used only by torpedo boats like the Schnellboote.
2) The T2 model of the G7e was in service with German U-boat fleets from the first day of WWII. In stark contrast with the G7a steam driven torpedo, the T2 left no visible stream of bubbles to alert ships they were under attack, and was virtually silent; however, these were the T2's only advantages over the G7a torpedo. The T2 in all other respects performed abysmally when compared to the G7a. Its range was much shorter than the G7a's at only 3000 m, and it ran much slower at 30 kt (55 km/h).
I remember when the T2 used to have a range of 3000 meters before GWX changed it to 5000 meters. I wonder why, when historically 3000 meters seems to be accurate? Now, the G7 e/T3 has a range of 5000 meters, but the T2 didn't.