1) What is the maximum real dpi for scanning (beyond which there is no improvement in scan quality, just increased file size)?
A full answer would take a textbook. In a nutshell, you need enough real optical resolution from your scanner to recover all the real optical resolution captured on film. Beyond that and all you are doing is making more data, not capturing more visual information.
Unfortunately it's very difficult to tell how much optical information a real piece of film contains. It's hardly ever limited by the ability of the film to record information; it's mostly limited by the skill and workflow of the photographer, and by the optical characteristics of the lens / camera.
Typically, as format size increases the amount of real optical resolution declines. The reason for that is that as format increases, capture aperture for the lens decreases. This is true because of the demands of depth of field, and it tends to push lenses into diffraction limiting.
This is acceptable because as format size goes up, the necessary enlargement to make a given print size goes down.
So... all you have left are generalities. And when faced with generalities, all you can do is test. You'll have to scan the same film over and over at different scanner resolutions, make prints from these different scans, and compare the prints. This will tell you when you reach "enough" resolution for your needs.
Sorry there's not a simple answer to that question.
2) what is its true OD? (specs say 4.0 but I don't know if I believe it).
You are wise not to believe it. But in reality all the scanner has to be able to do is scan your film. If you are scanning B&W film for example, the dynamic range of the scanner is just about meaningless if the scanner is healthy. Color negative is more challenging, but just about any scanner can scan the full density range of most negatives. Tranny film is where the fun comes. And either the scanner can scan your film or it can't -- and your shadows drop all detail and become black.
If you really want to know, scan a step wedge and see. I don't know of any that go beyond a density of 3.6 (the old Kodak density test strip or whatever it was called).
3) Are the Betterscanning sheet and roll film holders necessary if one was to use the included V750 Fluid Mount kit and only scan using fluid, not the film holders?
No idea. Don't have relevant experience.
4) What is the recommended fluid for fluid mounting
Aztek pushes Kami. I find Kami is too volatile for me. You have to work really fast or the Kami will have evaporated. So I like the Prazio Anti-Newton fluid. Much nicer to work with IMHO.
5) is 3mm Mylar ok for fluid mounting, or should it be 1mm?
What you need is a scanner overlay. Special mylar that's flawless and optically clear, and optically colorless. And of course, expensive (goes without saying I guess). The best source of that in NA is probably Aztek. Some of the pre-press supply houses might carry something similar.
