Upload & Sell: Off
Thanks for the input!
As it turned out, I do not have a mirror problem. I am terribly ashamed by my mistake, lol. Having kids running around all over, I popped into the basement, and mounted the Leitax mount - then quickly mounted it on my 5DII, which had the mirror raised (live-view turned on). Then I turned live view off, but the view-finder was still black - thus the mirror must have hanged I thought. Turned out I had subconsiously put on the front cap...
So I can higly recommend the Leitax mount: it is cheaper than most adapters, it can focus to infinity, has no mirror problem, and is rock steady on the lens..
Took the lens out for a test drive yesterday...