I have an exciting update!
I was going through all my wiring again since my MMU2S upgrade. The MK2.5S has two plugs from the power supply, but the MMU2S board needs one too so they include a power splitter. The directions for the upgrade say:
Follow the cables from the PSU and unplug one connector from the RAMBo board (both are the same).
But the image shows arrows unplugging the power cable which happens to be the heat bed one. Naturally I followed that photo.
Since my reboot issue is directly related to when gcode commands turn on the heat bed (determined by @foosel), I decided to try swapping the splitter to the other (hot end) power port. This has completely solved my issue and I can now print with OctoPrint!
I guess there's some power situation with the heat bed when a bit of power is siphoned off for the MMU2S board. Plugging in the splitter to the hot end port does not appear to cause the same reboot issue.
@Ewald_Ikemann if you have a moment some day, would you mind checking if your MMU2S power splitter is plugged into the hot end or heat bed port? If it's in the hot end side, maybe you could swap it to the heat bed port for a try. That way we can see if it reproduces my original problem for you and we will be able to report this to Prusa support for them modify the upgrade directions. I don't feel comfortable saying for sure that they [Prusa] should explicitly recommend using the hot end side for the splitter without first having some corroborating evidence... and obviously I don't have another MK2.5S MMU2S or know anybody else with this exact setup so it might be nice to give it a whirl for the community.
I'm going to try downgrading to previous firmware temporarily to see if this was also what was causing my slightly different issue. Also I recognize that others have started using this thread as a rally point for their similar issues with MK3 so feel free to continue your discussion despite it being a different technical problem considering you don't need to use the splitter on that printer.
My deepest appreciation to everyone who participated in this troubleshooting adventure, I am so excited to be up and running again!