I wouldn't worry about the relay board. Yes, it was made by 3DS but I looked at it and it seems to have all the right T's crossed if I remember right... good AC side isolation, different sides of the board have the ac and dc components, opto-isolated ac/dc separation ... so no worries there.
So basically my elevator pitch is "a drop-in replacement control board that allows the use of open source software to control a cubepro with no tricky hardware modifications". Marlin was just the firmware I've had a little experience with. Really, it's using similar hardware to some of the other 32bit firmwares out there so many could be ported over if someone was interested.
I like your idea for "preheat the oven" mode, I'll have to remember that when I get to the final firmware modifications. There will be a few "special" functions that won't be in other Marlin printers, so I'll have to figure those out when I get to them. I'll have several "Aux" ports (similar to other Marlin based boards) so if someone wants to add a heated bed or an additional temp sensor, that should be easily done.
The door isn't locked, and I don't think there's even a sensor. This could be determined through heat changes (i.e. if A power is applied to heater, the temp should go up by B degrees per second). It's a closed system of sorts, so fast heat loss would mean you opened the hatch.
I am planning on wiring up all 3 extruders electrically, but if there's any software hurdles I'll just focus on the two. I think the Duo was more popular than the Trio.
As for my location, I'm in North Dakota ... and because of the winter we're having, I had to put this project aside for the weekend so I would work on my other project ... a 48V homemade electric snowblower conversion. Batteries came from a hybrid Honda Civic. The donor snowblower is from 1979. Almost done, just waiting for a part for the snowblower transmission in the mail.