I will assume you put the insert in the magazine with the film roll facing the full roll icon.
As usual… the 600x magazines are the easiest things to use!
It seems strange that the wheel might be hard to turn, were both spools properly seated?
Yes, when it felt hard I made sure there was no problem, both spools were OK.
That does seem to point ot the insert rather than the back. It might be cheaper to find a used insert than trying to get it repaired
I have other inserts I can use, but the counter not resetting properly would also indicate to me that the magazine also has a problem.
BTW before someone asks: yes, I did properly put back the film stage after removing the DB and connecting the magazine.
I haven't seen anything branded Rollei Metric before, what would make it metric rather than something else?
The metric ones, used for official photogrammetry, were supposed to have tighter tolerances. I have not seen many 6000 accessories / bodies branded with Metric, but I have been watching the 3003 on the infamous auction site and a 3003 metric pops up regularly (those ones, except if modified, will, I believe, have the grid used for distance and deformation measurement somewhere in the body to get printed over the negs, and the lens screwed to some specific focus, probably infinity). At the moment, i see 3 Rollei 35 metrics, one 6006mod2 metric, and one 3003 metric.
If you look at the pictures of that 6006 auction, you will see the magazine does not have the yellow (for 220) or black (for 120) label on the back, but a red label (for 120), and also the gridded screen where the screwed-in film stage is shown.
Also, 2 out of the 3 Rollei 35 metrics that are sold have a Sonnar lens, and were made in Germany, without being branded as Rollei 35S.