iso – How to guess the specifications of a movie?

In an attempt to identify the type of "mystery film" I bought and mentioned here, I would like to do some tests.

1 – Can I take multiple pictures on the same roll with a different ISO setting on the camera, then see which one stands out the best? (Or should I shoot whole rolls with different ISO settings?)

2 – Is there a way to "guess" whether it's color or black-and-white by trying different settings in the camera?

The first roll that I shot was treated in black and white and came out very contrasted. I liked the results, but I am curious to try to test the film because I often buy unbranded / unlabeled movies.

Sony a7RIII vs Sony a7RII: specifications and prices

I'm thinking of buying a Sony a7RII or a7RIII and I'm just curious to know that the price of an a7RII is about 2/3 of a7RIII. Is the a7RIII really worth this supplement?
Is it much better?