The simplification looks good as you don't want to deal with meshing your threads for a setup like this. As a general thought you may want to cut the threads off rather than fill them in so it does not make the model artificially stiffer than it would actually be and err on the side of it being thinner/weaker.
For the fixtures, On Flat Face works fine for your cut faces. You would not, however, want to place On Flat Face on both ends of the model as it may change length as a result of the pressure load and you want to allow the model to be free on one end to allow for that length change. Having one side fixed just sets the zero location for that length change and itself is fine.
I've been off on a short vacation and am just getting back into things here.
First of all - thanks for your comments. Two stand out - not over-constraining the fitting and allowing it to stretch and erring on the conservative side by removing the threads and not filling them in. Both make sense.
The fixturing is tricky and is often the part of the simulation I spend most of my time trying to visualize and set up.
In the end (suggestions from another source) I artificially 'capped' the thicker end of the fitting in to supply a face that I could apply a force to. I used a reference geometry fixture fixuring some (not all) threads with no freedom along the axis (blue arrows), a cylindrical fixture constraining any rotation (pink arrows) and a cyclic symmetry fixture to simulate the whole part (threads were modified in order to use this option) (green arrows). The loading was applied as a pressure of 63 MPa on the end and the cylindrical surfaces (red arrows).
In fact, I did it a few ways to compare results and all were relatively consistent.
All the best.