Skirts on and off for testing should be OK, but I wouldn't go trying parallel parking with them on.
Any impact will tear the tabs off the fender. An easy upgrade would be to connect the four tabs to the skirt with a 4-bar linkage. The bars only need to be longer than the amount you need the skirt to move. So, 1 to 1+1/2 inches? Two fasteners on each bracket so it won't rock.
...if I hadn't lost this model, I'd go back and finish it.
About the 'virtual' boattail I suggest the Coanda nozzle. A zoomy example here:
embodied here, lower left:
If you're working with exhaust, a bell-mouthed exhaust tip in a concave stainless steel bowl in/as the truncation.