I have not read the whole thread but early on you mentioned that the unknown resistors were heaters. Heaters are "non-linear" resistors because they change resistance with temperature and are designed to get hot. That means their resistance value depends on the current and the amount of time it has been applied. The current supplied by your Fluke meter to measure the resistance will be much lower than the current you are using to measure the resistance in your Arduino setup. I would not expect the Arduino readings to agree with the Fluke readings.
As I said, I have not read all 90 posts to this thread so apologies if I am going over old ground but the first question to resolve is why do you want to know the resistance of the heaters? That will dictate the current and time for which it is applied for the test. And that in turn will determine what the best measurement circuit and Arduino sketch is. From what I can see, there is nothing wrong with your circuit or understanding of Ohm's law but you have overlooked the fact the unknown resistors are non-linear.
Jan