This is a first - an actual discussion on the forum!
David, am I right in thinking you've got some kind of premium membership of ridewithgps? You might find it easier to sort out the 'short reference names' than I did - I think I got there in the end, but it took me hours to sort out and I can't remember what I did now. If you're able to collect together the various routes and give them recognisable names, that would be really handy.
Re: two fast groups on the same route at the same time - that might work. Or, alternatively, the faster of the two groups sets off a couple of minutes later - by the time they catch the slower fast group there may be some people who will be happy to take that opportunity to switch groups?
I think the bank of club run routes would help - probably only need maybe 6-8? 50-60 miles covering a few different areas and elevation profiles. All the gps files could be made available in a zip file pack, with matching short reference names that you can actually read on your Garmin. You could then just drop the lot on your gps to have ready, opening whichever one is needed at the start of the ride. Could always post specials on the Facebook page for something different.
On the multiple 'fast rides':
If both did the same route setting off with 2 ride leaders at the appropriate pace (one race/training, one 'brisk'), you could have a brief regroup point about 5-10 miles in where riders could swap between groups having seen what the pace was like for a bit?