...As for the rim business, here we can safely lean against the soft cushion of fiction without needing to fear any razors. The real Milky Way is known not to possess any sort of a rim or an edge. The Trek Milky Way prominently has this purple thing they associate with / outright call the rim or the edge. The latter, fictional construct can lie anywhere we wish. And, indeed, everywhere we wish, because it's always dead ahead of our heroes independently of their heading or orientation, and indeed always a narrow horizontal strip whenever we get a visual. We might be forced to conclude it's in the eye of the beholder only, then, manifesting as a single-dimension ribbon or an edge despite by plot requirement actually being a barrier in at least two dimensions.
On the issue of long warp trips, a simple (!) argument would be to say that travel time and speed are in inverse relationship. The longer you run your engines, the more your speed drops, from the increasing frequency of pit stops required. Covering 500 ly may be ten times faster going than covering 5,000 and a hundred times faster than covering 50,000, in terms of average speed - meaning it's actually a hundred times faster than covering 5,000 ly, and a thousand times faster than doing 50,000, in terms of ETA to finishing line. Especially as your pit stops within 500 ly of Earth may involve manufacturer-approved spares, while those on the other side of the galaxy will not...
This works on short hops, too: going faster than Scotty recommends will blow up the ship and kill everybody, supposedly. But never does, supposedly because the cumulative odds go up with time, and Kirk always cuts before he loses. So even dashes like the one in "Bread and Circuses" might work just fine, without establishing anything much about the practical top speed of the ship.
The true relevance? Writer convenience - they don't have to do any math on short hops, and somebody else will do it for them if the plot involves long duration travel as a background issue (that is, something that won't take place within the confines of the episode of the week anyway, because there's no duration to spare).
Timo Saloniemi