Let me try and answer your questions:

1. As it appears from the description, arrows set the mentality of the player. Constant use of them (or use on every single player on your team) is not mandatory (and moreover is not recommended ). I'd say it should be a cherry on the pie.
For example, MC with an arrow UP will occasionally play the AMC role and vice versa, MC with an arrow DOWN will play more in the defense as DMC. You've got an idea, right?

Common sense applies. For example, there is no point setting an arrow up for the DL if his crossing skill is low. Also, remember that arrows will tire a player more (as he will effectively play in two positions.

So, if you ask me, I use arrows only occasionally and only for certain positions (and usually for players who have two positions already - DMC/DC; AML/ML; AMR/MR; ST/AMC).

2. Team play bonuses. Yes, now if you would like to have them at 10% you should train your core squad together every day.

Hope this helps.