You can pass the owner of the WP in the exec line with
this.
You can check the status of the unit that is the owner of the WP:
_stage = _this select 0
_unit = _this select 1
goto format ["%1",_stage]
#0
@ (! alive _unit) || (IsNull _unit)
do stuff
So that might be the correct way in MP, but still very sketchy. Also you can use format instead of multiple ? lines (goto format ["%1",_stage]). Not sure if a timeout in fact exists. Also using WPs in MP is very unreliable as circumstances might lead the player way away from them