It worked!
I thought (according to the table in
Features and Operation of the Ka Mount) that I would have to tape a more "complex" pattern on the converter metal, but on the first try, just to see if it makes any difference, just by taping all pins from the "*"-pin and away from the red dot was enough! (leaving the two pins closest to the red dot in contact with the converter metal)
When shooting in M-mode, now the aperture doesn't move, and when setting the same shutter time as AV uses for a given framing, it gives me the same exposure, indicating that it's now operating wide open as I wanted.
So now on to test/compare the IQ of K400/F5.6 wide open vs Tamron 70-200/F2.8@200 + T6-2X to decide if to keep the K400 or not....
(Sure, the K has the advantage of possibility to stop down which the converter combo can't, but the tamron can zoom as an advantage, and when needing 400mm, usually even more is even better so then I would still have the F*300+TC (600mm/F9) that can also stop down more if needed, so in the end i feel the IQ will be the deciding factor)