1) Not all Allied units have a nationality identified (e.g., Guards). It seems reasonable all such units should not be considered Indian, Australian, New Zealand, etc. However, are they considered British?
For withdrawal purposes they are not considered British.
2) When an event requires specific unit withdrawal (e.g., British or Indian infantry) is one required to choose units so specifically identified if they are available?
3) If the answer to 1 is yes, please confirm unidentified units of the appropriate type may satisfy British unit withdrawals.
Consider British units all units not of the Commonwealth. Check unit types symbols, Page 3, last lines of the page:
"Colored symbol boxes for the Allies represent the Commonwealth nationalities."
4) Can mechanized infantry ever satisfy infantry withdrawal requirements?
No, Mechanized Infantry is a "Mechanized" unit type, not an Infantry Type, as per unit types symbols, Page 3
5) When two Org Phase unit withdrawal events occur on the same turn (e.g., an event is advanced to the same turn as another) are the withdrawal events implemented simultaneously? If not, which is implemented first?
Yes do them simultaneously: Combine them in one withdrawal event.