Yes, Lens Sim (Full version) is designed to allow you to create and share your your own lenses.

Yes. Read the Render times section in the documentation for a more detailed assessment.

Probably? The main issues are with addons that changes the camera parameters, Lens Sim is setup to work with a specific camera setup; a specific focal length, focus distance, orthographic mode etc. If other addons changes those parameters the system will break. Also, having extra custom objects in front of the camera for vignette/bokeh effects is not going to work with this system. Other than those issues, addons should work as normal. For example; The Photographer addon. When choosing a lens via the photographer addon it changes some of the camera parameters, breaking the system. However, you can just press "Disable Lenses" and "Enable Lenses" in the main panel to reset the Lens Sim camera back to its design settings. I have not verified it myself, but that seemed to solve the main issue a beta tester had for the Photographer addon.

No, the system only allows for rays to be traced from the camera and out the lens, not inwards. Lens dirt Type 1 can receive and scatter light in front of the lens, but that's not real lens flares.

Yes. The python panel is only there to make it easy to use and alter the lens material, once a lens is created it does not need the addon to work. Make sure to include all textures used in your lens setup before submitting to a render farm.

No, it only works in Cycles, Blender 4.2+.

Yes. As long as cameras does not intersect(and lens dirt Type 1 is disabled) they will practically be invisible to each other. The overhead of having multiple cameras is one transparent bounce when a ray hits another camera. Unless your having multiple stacked cameras on top of each other the overhead is insignificant.