Maybe I've missed some information about the reasoning behind this, but I never understood the area around the forward spawn on 2nd. You can't stand on top of the roof above the forward spawn. To the right there's an opening where you can jump up, but you can't jump over the beam. Maybe you wanted to prevent players from standing on top of the beam, but in that case you can place a roof above the entire high ground in front of the forward spawn.
2. Add an additional path to exiting last.
I suggest adding a new exit from upper lobby out to the forward spawn balcony. A door here most importantly would not be easily controlled from the shack position, and would serve to help people pushing out of last to draw eyes away from the other main choke points.
Another option, could be a vent connecting through the large block here that leads out to the balcony. I could see it even being a 1 way door, only being useful pushing out of the building and not letting people in from the balcony, so that it doesn't introduce any issues with defending the lobby or backcaps.
I do like the idea of having a new exit from lobby to second, but I'm not really sure about a 1 way door. If the defending team tries to forward hold in lobby, you can have a sniper at that 1 way door so the only class that can deal with that is a demo with stickies.
Something I liked about logjam is the window/dropdown top-left. And I feel like something like that could also work on reckoner. On the picture below, everyone from the defending team can access this dropdown from lobby, while the attacking team can only reach it with a soldier or demo. This way the area in front of the forward spawn makes more sense with the invisible barrier above the beam. If the hallway to the dropdown is narrow, you can easily spam it so a sniper from lobby can get pressured.
But yeah this is just an idea, maybe there are some important issues about this that I'm not thinking off.