I think you should keep the double jump to the vent. That is a fairly cheap entrance and should be hard to get too. I think it should be even harder than it is. DoP had two people in there almost every round.
The problem is, if you make it very difficult and, say, only 2% of players can get into the vent, it makes it very easy for that 2% to grab the flag and severely knocks the balance out of whack. The other 98% have little chance against them.
All paths should be accessable to every player, and you should balance the paths out. Making it more difficult doesn't balance it out. It simply reduces the number of players capable of taking the path, which, in fact, does just the opposite of making the map more balanced.
Perhaps instead of having a trick jump to the ladder, you could have a path that requires the player to expose himself to the defending team and work his way around on the rooftops to the ladder or something, so it wouldn't be such a trivial drop down and grab.