1. The difficulty scales at the end of a hive. For example, if you have 4 players on hive 8 and someone leaves, you run with three people on a hive set to 4 player difficulty. On hive 9, the difficulty will scale to 3 player difficulty.
2. Completing missions solo does count towards mission completions (Not sure about the casual difficulty though).
3. If you are not downed when the help flies away or the breeder is killed (Scoreboard appears), the mission does not resister as complete. Technically, you "didn't make it", and the game counts you as having died or been left behind.
4. Yes. If I have access to three relics, but have only completed point of contact with one relic, then my max relics will show as one.