>>(5) if the close company is selected from the menu ( and it should be disabled ( = skip for) as long as there is a screen active
>> - clear the company context
>> - remove the application menu
>> - go to (1)
>
>The disabled part is crucial here - there should be no way to switch to company B as long as there's something open from company A. This needs to be clear switching, not parallel work.

Open form count helps here

>
>And even the application menu doesn't necessarily need to be rebuilt, unless it contains company specific items.

Oh, that is what I do - makes it easier to fine tune
Open : adding pads (bars, ...) to the menu
Close: removing pads/bars, … from the menu

And I keep the structure of the menu in a table ( several hundreds of lines ).. Menu designer or in code is too complicated