Did you miss the suggestion by the RMT where there would be no industrial action for 3 months if SASTA suspended their proposals for the same time whilst talks took place ? SASTA point blank refused, they couldn't even be bothered to turn up for a meeting with the RMT this week so why get ACAS...
I doubt there are many spare buses/coaches, a lot are contracted for P&R and Seagull Travel plus of course Southern using them already on the Seaford line etc