So seems like there are plenty of outstanding questions:
- How is Team Play Bonus gained (condition loss, certain drills, player morale...)?
These are gained by doing training sessions that also increase your Team Bonuses.
- What's the most efficient way of staying at 10%?
Probably clear from above answer, but I'd say treat this strategy the same way as you would with keeping your Team Bonuses at 10%.
- Does it actually affect game results?
Yep
- Does the Team Play Bonus apply to Association games too (I suspect not)?
Yeah, Team Play Bonuses apply to all matches, just as Morale, Condition, Team Bonuses, etc. do