Couple questions about MSUs:
1. Can an MSU which is unable to trace to a Supply Depot, Friendly Urban Hex, or Map Edge Supply source be used to avoid Isolation?
Yes. Although it can't provide supply, it's still considered a Supply Source for the purpose of determining Isolation.
2. Is an MSU that is unable to trace to supply source during Isolation phase itself subject to an ER check or surrender?
Yes. I'll clarify this in the NWP rules. So, the effect would be, units which would normally be Isolated are not because they can trace to the MSU, then the MSU becomes Isolated. In effect, the troops used up the supply represented by the MSU to avoid it for this turn.