What do you mean by rigged adaptors? Aren't they using similar plugs? Can't you just swap the plugs at the board so the bed thermistor goes to the hotend thermistor port and vice versa?

Anyway, sound like you solved it - short at the thermistor bulb. That doesn't solve the raise-stop-raise you noted in the original post, but that may have simply been a loose connection, which you have now reseated. Hopefully your E3 is now working and we can look at what's going on with the 4.2.7 board.

One last thing, you mentioned two different thermistor types, cylindrical and bulb. How is your hotend setup? Does this mean you're out of action waiting for a suitable thermistor.