I read Amane's comments on the impacts of captains on team play a few months ago and since then have been paying close to attention to selection and performance of captains.
I read that too. I tried in another team I have in lv1 (for friendlies and experiments) which I 'm struggling to keep it in the same lv.
(did that last season)
Tanking is harder than you think, specially in first levels playing against newbies. So I thought instead of using illegal formations and players oop (with the danger of cards and injuries) to set my captain the worst player. Didn't help
Nothing like using as GK someone else oop (but if your oppo does the same, no result).

Amane, if you want, you can try this experiment with a new team.

In order to keep a player in low form, he must keep playing as much competitive games as possible so that there is no chance for his motivation to recover.
Some more tips from my experience :
-There is no (hidden) motivation.
-Bad form of a player is random and can hit like a disease any player of the team. This has nothing to do with playing many games. If a player is giving many matches then his form going (naturally) as a wave. Some games a 8, some 7, some 6 and so.
-A rate of 6 isn't so bad for a GK ,DMC or MC but it's really bad for an AM or ST.
- I had players that played 3 seasons without being in bad form (I mean continually rates of 4 or 5) even if they participate at most of the games. That's why I use only one GK every season. There is no need if he is good player, fast trainer and being in a good team.