Campaign code improvements and fixes

[speed2CZ]

Still WIP, I'll include full description later.

Description of the proposed changes

• Collection of fixes, improvements, refactors and new annotations all concerning coop / campaign code.

Testing done on the proposed changes

• Ive played through couple of the missions to check if or how the behaviour changes.

Additional context

Checklist

• [x] Changes are annotated, including comments where useful
• [ ] Changes are documented in a changelog snippet according to the guidelines.
• [ ] Request 2-3 reviewers from the list of reviewers and their areas of knowledge.