Have a look
here. Though they deal with B737, events 5 & 6 are of some relevance to thread.
FL390 is pressurization limit on 320family. If you exceed it, cabin alt will climb as the pressurization controller keeps max diff pressure but that's about it. If sector length wasn't a factor, I never had problems reaching it on 319 with CFM56-5B engines. 320s with -5A were somewhat different story, cruising at FL350 when heavy. Vls is about 1.28 Vs1g when clean, so bus doesn't stall immediately below Vls mark - its the black/red stripe on speed tape that shows your stall margin, not orange one.
I believe the Max Ceiling is pin programmed on the TCAS Processor mounting tray at installation.
Definitively dependent on aeroplane and TCAS computer model. See link above.
The limitation stems from her (in)ability to get down.
Hmmm... I really doubt it. There are no speed or alt limitations on use of speedbrakes and with boards out at Vmo, she dives almost like SB2C. There is a recommendation about not using speedbrakes above FL250 IIRC, for pax comfort though.