Glad you're making some progress. Still, something odd is happening with your set-up.
I think you've solved or at least are on the right track with the crosshair movement. Calibration within Windows is the first step and can be tricky. Don't move your wrists when pointing at the three calibrations targets on the screen. Only point the barrel of the gun if that makes sense. That will yield the most accurate and smooth calibration. Stand back from your screen at least 4 feet when doing this. If your guns then work as mouse pointers in Windows, you should be good to go. Move onto to MAME (but not Hyperspin yet...that's the last step).
I'm not sure what to tell you about the P1 & P2 button mapping. Getting something like "Gun 1 Button 0" or "JOY Button 1" is correct and should work. Don't point your gun at the screen when mapping the buttons. You DON'T want it to pick up any X/Y axis coordinates along with your selected button(s).
If both your trigger button and your rear "reload" button map the same button, something is off. Sounds like you already checked it out in the AimTrak utility. That's what I would have suggested. Maybe you go back there (configuration tab) and change it up to be sure they are separate buttons? Write down what the buttons are supposed to come us as and then cross-reference those button names to what comes up in MAME (or should be coming up). Be sure you are NOT moving your mouse while doing any button mapping of any kind. keep it on lock-down somehow so it doesn't get accidentally picked up.
When mapping a gun button in MAME: select the button in the menu, press ENTER, then press ESC to clear ALL entries. The mapping should now say "None". Then go back to the button, press ENTER again, now pull the trigger or press the gun button desired. That's the best way to map gun buttons (for me) so there are no conflicts or additional unwanted mapping entries.
Mmmmm... I may have to think a bit more on this. Have you tried contacting AimTrak/Ultimarc directly to troubleshoot? They may be better equipped at this point to help you. I know Andy (Ultimarc) and could give you his email address. Or are your guns Arcade Guns (also AimTrak tech)? I also know Eric Harbo at Arcade Guns.
Another advanced idea: I don't know how computer saavy you are but the thread below talks about a utility that is used to automatically map ALL buttons, joysticks, AimTrak guns, triggers, etc each time you boot up your MAME system along with remapping the IDs correctly when/if Windows decides to change them. This requires some XML coding and a knowledge of what your buttons are supposed to be. The main obstacle this utility overcomes is the Window's ID issue (separate from your setup issues) but it can also be used to correctly map all your buttons and X/Y axis for ALL of MAME perfectly every time. Not required, but I use it. See my comments toward the very end of the thread along with code you could copy and paste. Maybe this will help you...or maybe just confuse you more at this point. I don't know. Thought I'd just throw it out as an option:
http://forum.arcadecontrols.com/index.php?topic=108767.0