Second problem about the scratching partly solved: I've installed another DS crank and with this one the clearance stays more or less 3mm all the time and it runs smoothly, without 'wobbling'.
So, one of the three arms of the spider is a bit out of place. Since the crankarm is useles anyway, I'm gonna try to 'replace' it with a hammer and vice tomorrow. Otherwise, i'll have to find another crank!
And again, thanks for the infos. I didn't studied a single letter for my exams today but at least i've learnt how to remove cranks and revise bottom brackets