Both of those scenarios should be able to keep the team on the right side of the bubble (though the second might require a win in B1G Tourney to be safe).
If Illini go 6-6 the rest of the way, which includes an improbable upset at Purdue, I would think that that would almost put the Illini in a near lock position, regardless of what happens in B1G tourney (unless it is a loss vs MN/NEB). I still think 20 total wins should be considered complete lock territory, but as you indicate, 19 wins with a huge road win against top ranked (assuming they remain #1) Purdue would be almost as near a lock as possible.
But I say, let's get to 21 wins in reg season (8-4 the rest of way) and remove any question of doubt. If Illini go 3-0 the next 8 days, then this has a real shot. I still think getting to 19 wins that does not include a loss to MN/NEB should still be in pretty good shape, but winning one in the B1G Tourney might be needed to ease any sweating on Selection Sunday in that event.