As a follow-up to the previous, I majestically failed with the other side's line, by cross-threading the line fitting into the junction block:

Notes to self:
- Unlike the M12 fitting, the M10 one will cross-thread VERY easily
- You can't really tell whether it's just your brake line bending the fitting slightly, making it a bit more difficult to spin, or whether the fitting is eating up the threads. The brass is super soft
- Just remove the junction block to enable easier install, or at least remove the lines from the main cylinder, in order to avoid flooding brake fluid. Trying to install the fitting with everything covered in fluid won't allow you to feel the threads.
- Create a "Lessons from redoing brake lines" thread or a tech article once this saga is done and the Bird is back on the road
Well at least there seems to be enough material to make this side M12 as well now, gotta take it to a shop and ask if they could create me some new threads. I don't really want to mess this up much further, and I'm really screwed if this junction piece is Fucked Up Beyond Any Rescue.