My 2 (euro) cents on this topic.
As you say, the UMO+ uses a v2.1 board, which has the electronics in place to read temperature from a PT100 thermistor. Using another thermistor or a thermocouple won't work.
The easiest is probably using a free analog pin from the micro-controller (E.g. from Exp.4) and re-assign the pin in Marlin.
Wrt the thermocouple amplifier, AD595 and AD597 are similar in term of output voltage curve, the difference is in the 'preferred' ambient temperature (the AD595 performs better at ambient T° around 25°c, and for the AD497 it is 60°C). Apart from that they are interchangeable and you can still select AD595 in the Marlin build.
I can't comment on the 12V cartridge (I have a 24v one, but still on the workbench )