A reason for the fan being under the rocket could be to keep it cool. Red was having a hard time keeping the rocket cool with the engines running all the time. So they put a big powerful fan underneath it.
Nice work so far, but I don't know if it's just me, but B2 does seem a bit more laggier than B1 did. Be sure to fully optimize it for the next release.
Also, another small choke point is right after Point 3/C, there's a small ledge where you can stack up about 5 sentry guns. If all 5 guns are up with an engineer on each, it's nearly impossible for the cart to be pushed forward. I'd propose you make another ledge behind the rocks so you can look over and blow up the guns with sticky bombs and rockets.
I'll be uploading the map to our server shortly and we'll see how it goes with 24 people.