Regarding the Summit, the NCAA removed the continuity rule for conferences. So if the XDSUs left, they'd have:
South Dakota (2011)
Omaha (2012)
Fort Wayne
IUPUI
Denver (2013)
Western Illinois
Oral Bob (2014)
North Dakota (2018)
Tight, and they may need to kerjigger the sports each school offers, but I think they can get by. They'd probably try to poach New Mexico St, Utah Valley, and even Grand Canyon if possible, or even a full-blown merger with the WAC just to survive if it got dicier. I suppose they could always ask UMKC to come back, and threaten them by taking their conference mates instead.