The easiest fixes for 2fort would be to close off the extra resupply room in the basement
That still isnt a good fix. The reason 2fort has problems is because you have to get past a area just outside of the enemy spawnroom twice!
The main spawn exit can reach all primary paths in only 512HU. 1 sentry can overlook both entrances to the intel room. This is a bottleneck that is simply hazardous and the way to fix it is by being able to flank past them. And this is especialy noticed because the respawntime is faster than it takes players to reach the intel and walk back. So you have to face the same enemies twice.
For that both the main and basement spawn should be removed so you are only left with the battlements spawn, or the 1 way exit. This instantly prevents the main enemy entrance from being reached fast (the outside area on the right of the main spawn). But this still will not fix the '1 sentry can overlook both'. It only makes the walk longer to reach it (by only like 2 seconds when creating a direct path, or by about 10-15 seconds when not having one). However, as there is no infinite ammo supply there, getting your nest on that exact spot can be made harder. And to at least provide ammo during calm moments you can place a large ammo pack at the bottom.
Note that even in such case the sentry nest can be a major hazard, put when combat goes on for longer where the engy has a hard time to reach metal you are more likely to overtake them. This is shown on multiple valve maps where for example the gravelpit roof is many times camped by sentries and dispensers. You can create that same situation. And ofcourse, underskilled players cant take it out, but for good players it at least doesnt get into a frustration point. And once taken out, engineers cant take a direct path to that area. they already need to get past the battlements (risky) or take the same detour as attackers would take themselve. Once the area is lost you suddenly become at the disadvantage there.
The center of 2fort, even though its a sniper fest, isnt that bad. A decent player can quite easily find ways to get past it, especialy since even in a 3v3 sniper battle, they distract each other enough. But to be able to get out of the water will still be a welcome addition to prevent the long sewer path from becoming a pain. But those slopes in those videos are just ugly and feel completely out of place. For that widening the water section and having the slopes go in a L shape could already work nicely. or maybe just have the ctf_turbine approach of slopes. Never force players to take a sharp U turn on slopes, it forces them to look at the path more than at the combat that is going and this isnt a good thing and only anoys players when they die.