I've no experience w/Montessori after waldorf, but I will say that we looked at a number of schools before settling on the pub. school for one, homeschool for the other combo. We weren't especially going for pub. school, but the absolute clarity of the teachers, the happiness of the children, the lack of kids sitting outside the classroom, just to name a few drew us in. And, as I've said before, post-waldorf kids and families were a known quantity and there was a clear system to remediate the academic weaknesses. It felt like we'd been viewing everything through gauze-never any clear answers, psedo religious answers, class teachers overstepping their bound or assuming knowledge that they never actually had, and then suddenly the gauze was lifted. We were clearly looking for a place that had a no tolerance policy for bullying because we'd frankly had it up to here with one of our children being hurt and the "karma" explaination just didn't fly any more. The kids at the PS were so well supervised, play was pretty harmonious and non-conflictual. In short, it was what we had thought we were getting, but hadn't.
But, it was still tough to admit that waldorf wasn't going to work. The promise was there, but never fulfilled, as you hear over and over again. It's very, very, sad. As an FYI, I have seen the posture your son describes used with kids, although they were older, and I have heard a waldorf teacher describe the reasons why they would make a child stand in this posture-it's supposedly some sort of therapeutic posture, if I remember correctly. If Bean sees this she may have some light to shed.
Good luck. I am sorry for your experience and I hope your son ends up in a loving place.