Sounds like you have the lowdown on the place. I have seen quite a few ads for Siam, but it sounds best avoided.
I worked for a time at one of the 'Big 3' unis here. Even in the 'best' places there are plenty of problems, so God only knows what the lower-rated public or 2nd division private unis are like. I was once offered a Rajabhat job, although fate stepped in and pulled me away overseas.
In my previous Big 3 job.......
Farangs had no say in management. The Thais were terrified of farang experience and expertise, but couldn't manage a noodle bar by themselves. So, there was always some mutual suspicion and paranoia (despite the large dollops already there because of the Thai tendency to form factions and in-fight) Unfortunately, this type of culture does not lend itself to positive development and evolution, participation, and learning from mistakes. Quality-driven teaching is severely restricted, as morale is low, and turnover is quite high. Some dedicated farang and Thai teachers work themselves into the ground and burnout, carrying the others. Sounds like the situation at Siam.
The 'up the as...' sounds familiar. In my old place, the Thais were often Associate or Asst Professors, which is really a joke, but they liked the titles. Senior native speaking farangs, often with several degrees and 20 years' experience, were never allowed the titles, although some had 1000 times more knowledge and ability. Younger Thais with PhDs, often based on obscure research at 'lower'-ranked US universities, would swan around doing very little apart from messing up and then relying on farangs to clean up the mess...or they would ask farangs to edit stuff for basic grammar and spelling mistakes. Older Thai teachers, mainly female, would arrive in CL 500s, show off their diamonds, gossip over noodles and then depart. Whether they actually taught anything was a mystery to me. Thais get very easily fazed by a large workload, or by having to use several parts of the cerebral cortex at the same time.
One or two of the farangs were like the Walloon you mention, but slicker. They seemed to balance all the plates by sucking up to the inept Thais and showing their faces in the right places, although they were more into looking at boobs and skirts than the whiteboard.
M