I agree completely that the club needs to make the small number of general admission tickets left more accessible to non season card holders. The adult ticket price is generally £55-60 for single games which is way too much.
Like you say the best opportunity for new fans to come to a game are usually CL group games and cup games where prices are cheaper and there's more availability of adjacent seats. Unfortunately these tend to be the games where the atmosphere isn't quite up to par which I can imagine doesn't create the best impression.
Regarding the OSC coaches being made up of mainly 50+ year olds, could this not be more to do with the OSCs themselves (for whatever reason) failing to appeal to younger fans? It seems to me like we have plenty of younger (under 30s) fans at games but would be very interesting to see the data on the average/median age of fans over the years.
The younger lads prefer to travel independently home and away.