I'm looking for a definite answer for what happens when in a mangle subchain called from Prerouting:

- Mangle -> Prerouting -> ChainA

If a rule in ChainA matches and it has Passthrough = no, will it return to the main Prerouting chain or will break the Magle-Prerouting completely?

from documentation
https://help.mikrotik.com/docs/display/ROS/Mangle

passthrough (yes|no; Default: yes) whether to let the packet to pass further (like action passthrough) into the firewall or not (property only valid some actions)

The way I understand passthrough=no will stop evaluating further filters (for that package) for the mangle process.

Here it is explained a bit better:
viewtopic.php?p=733485#p733485