Thanks. Surprisingly I could make a fix already, and a pretty simple one too.
One backroute on Picard involved bashing into the steel blocks, which in turn, were added to remove another backroute of a different nature which I don't remember so they couldn't simply be removed. Besides these backroutes there still were fundamental problems with blocker behavior.* But in any case; when it's reached this point I 've decided to scrap the level and make it totally different. I don't think a simple fix will fix it; I don't feel like making 10+ amends to a level anymore. [of course that will probably happen to this level too]
Now I've managed to do it with no blockers, and no builders, which is nice.
I discovered this a while ago but I don't think I said anything because I couldn't think of any way to fix it at the time.
*Of which, was partly taken care of in Lemmini due to it's poorly designed blocker behavior, until RTW proved that didn't work either. But I don't like making levels relying on that sort of thing.