corporate-wage-slave

Conformance can be as low as 22 minutes. After the cut-off, if you are HBO, then you can ask the staff to let you through, they will need to ring the gate and then FMU, but it's a routine thing - several dozen people a day make use of this. If you are a large family group or look less than mobile then the agents will probably encourage other options. Conformance is now switched off when there are bad delays.
Ground agents will tell you the gate if you ask nicely. Obviously going to B or C early is a risk (mainly on shorthaul) unless you know about the passageways. You suggested that the FIMs are only updated at the last minute - it's usually 75 minutes (and sometimes 120 minutes on the "will depart from B" message, but particularly for longhaul the agents with tablets or PCs will normally indicate the expected gate.
Almost all point to point shorthaul tickets, in both CE and ET, now have SDC, with the notable exception of Basic fares. If there is space, you can fly early or late. For longhaul you would need a flexible ticket but not that many longhauls have such a large number of flights that this is practical. However it can be done, even with Avios tickets (for a £60 charge) out of LHR.
Normally BA tries to keep people off standby lists, so the SDC mentioned above should be seen as the main conduit, however if you are HBO then BA will put irrop passengers onto Standby and you can plead your case in other circumstances. It can be done.
Ground agents often shuffle people around if the flight isn't full, but the big difference is BA's use of Altéa's Theoretical Seating algorithm. The complaints about split up families - which used to be commonplace in this forum - have almost evaporated, I can't remember a recent bad case of this. Sure it still happens, particularly on irrops, but to say ground staff are powerless simply isn't true.