The ladder thingy I always thought was an emergency support strut to keep the roof up (or the floor down). I can't remember is it was labeled at all... Still, it could be somethign that comes up or down for emergency egress.
I recall an "EMERGENCY USE ONLY" sticker, the only legible piece of text on the set, being more or less in focus when our heroes first enter.
Which is too bad, because the bridge dome of the ship is of a somewhat exotic type in comparison with the Constitution
ones, and might warrant regular access to an upper level...
I'm not sure I'd let production concerns influence this, as production also dictates the set had no forward half but we still have to postulate one for the "actual" bridge. But point taken: the set probably did not feature a third such pulpit.
There are rather few chairs there: even the TOS bridge had two seats in addition to the CO chair and the sextet (here just a duo?) of background chairs. An additional one would be rather welcome, assuming the ship still required Helm and Navigation specialists, or even if only assuming the ship had Conn but also Ops and Worf was neither. Only one of the background consoles appears properly crewed during the wargames...
The whole bridge module was cobbled together from whatever was lying around the supply depot or wherever the ship was sitting
Makes some sense: we already have reason to suspect that, in-universe, these TNG pulpits are modular units that can move around between episodes and aren't attached to the floor particularly firmly even during a given single adventure. Slap some in place of a lost or destroyed Helm/Nav console, and voila.
Incidentally, is this the reason the two chairs on the background appear so infernally uncomfortable? Were their cushions ripped out or eaten by Berellian Styrofoamworms? Them being original gear or off-the-shelf antiques in their normal condition, in-universe, is a bit dubious...
...As is them being actual furniture ITRW. The ejection seat observation sounds very plausible!