Mmmhhh... I'll never understand the "logic" of this game.

[kebess]

From what I understand, mentality affects your team's positioning and behaviour towards your opponent right ?

So why is it bad to play "Attacking" with a non offensive formation ? I mean, your team can still assume an attacking behaviour, and positioning, right ?
For example your defenders might defend a bit closer to the midfield line and would even cross that line on occasions, if a pass is less effective (a bit like what my defender did, in the match I described in the OP). Your attackers will basically sit in the opponent's goal and wait for passes, and midfielders would play a bit higher and press towards the AM section.

With a defensive formation, you'll have more defenders obviously, but why should that necessary be bad for an attacking mentality, if things works like described above ? All they'll do is try to press more when they have the ball, by helping the DM section and even the midfield area.

When they don't have the ball, they won't just stand around, and wait for it to come at their feet. They'll naturally resume a defensive position and try to get it back, but as soon as they get it, they'll resume the pressing yet again.
This is how I picture it from your description, so why is it bad to play offensive with a defensive formation and vice versa ?

Sure if you play offensive with an offensive formation, you'll have more players doing the pressing in the opponent's half of the field, but like you said, you'll also be more vulnerable to counter-attacks.
However, with a defensive formation, although your pressing would be less effective, your defense would still remain much stronger, even though you have an attacking mentality, allowing you to block much more counter-attacks. Don't you agree? That is not a "disadvantage" at all. It's just as valid as any other formation.