WW - Island Control
It's possible to start a WW on any island as long as all cities on that island are controlled by your alliance.
To activate a WW island - "alliance has complete control of the island on which they wish to build their Wonder" means u must own (The Alliance) every city slot on the Island for that Island to be eligible to build a WW. Hence if a city gets taken from an alliance that is building a WW and unless that city is retaken by the owing alliance then the building of the WW is stalled and no new resources can be sent to build up the WW.
Unfortunately, that is the rule.
Cheers
The Covernant