So in the end, Milim initially not participating was just a contrived way of displaying the power gap between her and everyone else.
How creative.
Because it's not like the stakes were high, such as the entire country Rimuru built up and the lives of all his followers.
This was clearly not something he should have asked for help with right from the start. I mean, his subordinates really believed in him, so who's he to go against their will and request assistance from Milim?