Try youtube for the video, you can set it so that only people with the link can see it if you don't want all of youtube being able to find it. :)
With the chair seat and thrusting, first off, get a professional to check the fit of your saddle. I had chronic chair seat for YEARS and then had a saddle fitter who also does bodywork fit my mare's saddle. All of a sudden, no more chair seat. I still thrust but nowhere near as badly.
Try to engage your core fully. That is, tuck your bum under you, try to touch your shoulder blades together, and pretend like you're touching your belly button to your spine. You should feel your back AND your tummy engage, and your obliques too. 90% of riding comes from your core (the other 10% is legs). Babies mess with core strength so don't feel bad if you can't hold it long or don't get it right away!
Pilates (especially the plank) and yoga are brilliant for balance, flexibility, and core strength/stability.
I've also noticed that with my core engaged it's MUCH easier to keep my hands where they should be.
I would say the majority of bad riding habits in ALL people come from lack of correct core engagement. Including my own bad habits.