Going by RAW one could interpret the rule on p.277 in a way, that the team leader must pass a willpower check for each individual battle brother, encountering a fear causing creature and being in squad mode. Although cohesion damage from fear occurs only once per encounter, the chances of failing the check increase with the number of Marines affected.

Personally i am inclined to handle this as just stated, especially considering, that the fearless talent helps in that regard, as one would not have to check for the fearless character, otherwise handling of a mixed fearless non fearless team becomes unclear.

What are your thoughts on that ?

Pretty sure it's intended (isn't it written in there? pretty sure I've read it somewhere) that the team leader makes the check for the whole group (every turn until failure, and then no more?).

Aye, it's not for each individual in the squad, but the squad as a whole. You only make the one roll for the whole unit.

Makes fearless quite useless in that regard then, hm perhaps a +10 bonus to the roll, for each fearless team member

Yeah, Fearless is pretty bad. Only ever really useful for solo mode, and then you still have drawbacks.