1 - Is it possible to force a staircase to the front entry even if your software thinks there is a jetway (while there is none, maybe due to a bad design of a freeware airport)?
Of course there is, the "Parking has a jetway" option, explained in the GSX manual, Page 24:
Parking has a Jetway
This option can be used to disable the front/middle Passenger stairs on sceneries that have static Jetways or use any other method than the FSX native one to display or animated them.
In your case, the jetways HAS been detected, because it's in the scenery AFCAD so, by disabling the option, GSX will display all stairs and the passenger bus.
However, you might want to check why there IS a jetway, but is not displayed.
It might be or not a problem of the scenery, maybe it's programmed to appear only with higher Scenery Complexity settings. Or, GSX might using the default AFCAD for the scenery, which has jetways, but the addon scenery has disabled them and its own AFCAD is not setting them OR the addon AFCAD cannot be used by GSX, because it's only an enhancement of the default one, so GSX is still reading the default one.
2 - Is it possible to stop the messages during pushback? I can hear them clearly, no need to read them too.
The "Verbosity" slider in the "Simulation" tab of the GSX -> Settings menu.
3 - Is it possible to disable the numeric display above the marshaller for more realism?
The "Marshaller distance" readout in the "Simulation" tab of the GSX -> Settings menu.