After your post, I realized I misunderstood your suggestion, Alias. Here's what I ended up doing (but I think this will still work nicely and interestingly!):
Dropdown at RED's last
...which leads to here:
What I've done here is provide two exits for RED, one on either side of the final bulkhead. The fence is to prevent BLU from shooting up into RED spawn. Using light to direct RED to one of the two exits. The door on the left in this image has a respawn room visualizer inside. The respawn room visualizer for the other exit is perpendicular to the viewer, going between the support beam to the wall with the arrows/red exit (this is so if a sniper wants to sit in the door, they won't be safe behind a spawn blocker).
Your suggestion was that BLU would be able to use this route around, so that's what I misunderstood. BLU would not be able to use this as a flank.
The door on the right goes:
The door on the left comes out under the stairs:
BLU has access to this room behind the fence (and I moved the small healthkit + med ammo pack inside to favour RED for purposes of defense). This exit closes after BLU opens the bulkhead for the purposes of preventing RED from getting an easy flank behind BLU's on the cart.
Adding this also helped brighten up this dark area under the walkway.
This one shows the improved sentry platform on the opposite side of the warehouse (old one was frankly useless, even with cover -- everyone could outrange it). All I did was reverse it.
Also the raised door that closes once the bulkhead opens. Closing this door with no other changes should improve that sentry spot that faces the last point considerably (previously everyone could just spam it out through the door with very little risk from the sentry itself), while leaving it as an okay-ish spot for facing into the warehouse before the bulkhead opens.
And the other rather significant change...
Improved Defendability of 3rd Point
3rd was increased length wise by 256 HU and the point moved forward. There's also a "mini-spawn" behind 3rd which is active from the time 2nd is captured until the BLU pushes out of the other kennel gate. This should be just enough time to get one respawn wave for RED players who died at the defense of 2nd. There is no longer a forced respawn since this new mini spawn puts them right on top of 3rd.
Spawncamping will not be an issue here since by the time the cart enters the mine, this spawn will have deactivated.
Hopefully that woodpile does not prove to be a super sentry spot... it's meant to be good and there are plenty of angles from the side flank to take it out indirectly, but it's going to be strong against the main entrance to the mines now.
This version is fully compiled but not packed up. I'll do that tomorrow after I sleep on it, make sure there's nothing else, and then pack & upload to the server for some impromtu testing.