I have finally identified the source of flex and eradicated it...now I need to correct the spacing between CCD and the flattener (there is some field curvature in the corners), but otherwise everything seems to be going well. What a relief, it has been a long journey...
Out of curiosity, and to see how well will the CCD handle longer exposures, I tried a single 30-minute exposure through H-alpha filter. The resolution is 1.34 arc seconds per pixel, simple stretch only.
The result, I think, is not too bad
http://www.astrobin.com/full/178552/0/