MOSFET: Very high Rdson
I can't answer questio 2n since my computer got a problem and I can't see your schematic but I can answer question 1:

Q1. Am I measuring the Rdson correctly? (it does not need to be accurate, good approx is enough)

Answer: No you don't. A multimeter is not meant to measure a resistance if there is current flowing through it. The resistance measurement need to be done without power on the board.

If you want to measure Rdson you can do the same operation but putting your multimeter in voltage mode. You measure the voltage between the drain and the source. With another multimeter you measure the current flowing through the transistor. then You do R = U/I and you have your Rdson.