It's very unusual. Usually South Africans get at least 6 x months on the first extension of stay and 1 year thereafter if they are satisfied you are here for the correct reasons. Picture with you teaching and at school etc all totally normal
Noone can predict it.. But yes it depends on the Officer on duty at the time. They are watching South Africans very closely for back to back stays and different visas but not the correct ones to be in Thailand long term. The visa issued at the consulate doesn't guarantee his entry back into Thailand. Its a leap of faith.
as with anything in Thailand its unpredictable. You would need to make that decision yourself. Perhaps ask the 2 x employers to connect and failing that then exit
it's quite a process as your employer must cancel everything timeously with your new papers already being ready to process as you literally have a 7 x day window to complete everything once your current documents are cancelled. I would ask your employer to perhaps not cancel anything until you are in your new position and the new paperwork is almost ready.
We have had teachers who manage to do the whole process without leaving Thailand again but it is entirely up to your current employer and so it depends on whether you are leaving on good terms or not. Try to discuss it with them. Its definitely possible but the decision lies on the employer whom your current documents are done through.