Huey - I'll address a few of your questions as best I can.
First, my airbrushing amounts to precious little amount of paint going on to anything other than the subject, but still, in time I do change my filter as it does have some spray discoloration on it. I can detect no difference in air volume passing through the filter, so it's by no means at all restricted with accumulation of paint particles, more than anything I just don't like the appearance of it by then. (Fussy old bugger I am.) So yes, a filter does indeed help keep particles from entering the fan and other ducting, plus it restricts particles from entering the atmosphere.
Second, my light source is at least 24 1nches above the actual spray area, just a common accountant style adjustable position type, sorry, that's all I know about it. Zero ignition problems in many decades of use, with lacquers, enamels and acrylics.
Third, my fan is self contained in a steel box, same power and air volume movement as a utility room ceiling fan, specifically fume explosion proof, bought at industrial supply shop, not hardware store. The structure of my booth, (home made,) is such that I can no longer see the data plate of the fan, but with my description I hope you get the idea of it's power rating. For reference, my booth measures W18" X H18" X D8"
Fourth, my fan is mounted on the back side, with aluminum four inch tube exhaust duct allowing air to depart the shop, through a sealed and closeable wall exit. I can't think of any reason to have done it differently in my application, but see no reason that it couldn't. A bottom mount would of course require the booth to be elevated, for proper exhaust ducting.
I use 14X14X1" common household filters, available at about all hardware stores, see zero evidence of anything getting past it on interior structure. The filter is mounted on a slant, top is toward the booth front. Fan box then mostly fits inside the booth structure for compact size, but enough protrudes for servicing and four inch ducting tube to be attached/removed as needed, since the booth is portable.
I line the spraying area with white glossy paper for keeping it all clean, but so little gets deposited on the surfaces that it lasts for months with LOTS of use, that describes how efficiently the air and overspray particles are drawn into the filter, for exhausting outside. I only airbrush, if one uses spray cans with considerably more volume being applied, then you could expect more overspray getting on the interior surfaces. But taping paper to the walls and base would help a lot still.
By starting the spray off the subject, you avoid getting a blast of paint when you start and the airbrush or spray can is not yet in motion, continuing past the end does just the opposite. It actually is an important truism, just ask any painter that does 70 million dollar Gulfstream jets, and charges upwards of 100 thousand dollars to do it. Your only other choice, is to make sure your hand is moving before you actually activate the spray control.
In order to check my spray pattern quality, I save the shiny surface cardboard that comes on boxes from the store, I mostly save white, but any dark colors are also helpful for checking on the light color paints. I cut them into about 4" squares and hoard them away, priceless for checking my spray pattern and flow. Then you won't make an un-necessary mess in the booth interior, once you've used up a square, just chuck it.
I do hope you find this helpful, at least in some small way. Best of luck with your project. (By the way, my booth is the product of three prior efforts.) I always found something that would make it a better size, lighter weight, portability, etc.) But it was a really enjoyable project, I'm very happy with the end result.
Regards
Patrick