investigate shisetika or michief and get the mana team members?

do you get forced into looking into it like with some other quests or does something go horribly wrong and they end up dead?

and i mean any of the optional investigation quests, not limited to those two.

Some of the optional ones just remain open forever and some of them are required for you to advance your guild rank (depending on what mission it is)

I believe most of the ones which require you to recruit members are required to advance ranks so they will never die even if you advance a few in game months.

You won't get those members, and probably won't hear of them again.

You won't get Asmodeus.
And lot of item(e.g. clothes) that require you to have them in your party.

so not getting them doesnt really change the story? lame

The main story does not change, yes. Why do you find that strange?
Although routes have requirements on recruited characters - for example, Yuela's requires Uya and Melo to enter, and Haku and Eliza are mandatory to progress.

And yeah, missing anyone locks you out of all NG+ mines content.

yes i do. the story always seemed to be focused around how will can overcome anything because he gathered all these races together as his friends.