sjk, welcome to the forum, this thread's a hoot! (I love the comment on drinking and putting up the fence, I'm the same way with sheetrock )
What you should be able to do is fill the pool to just under 1/2 way up the light, install the faceplate and cut it out so that the electrician can get to what he needs to. At this point, the water should be high enough to do the returns (but not if they are too close to the corners - if the corners are 'loose' ie, liner not tight on them, and the return is within ~5', you could develop a huge wrinkle by putting the faceplate on without the water being all the way up) [there is a way to do it but you'd need 2 people]
As for the green water, put some bleach in there, circulation is a good thing but clean water is more important - just add it to the stream from the hose, it'll mix up enough.
If you think a wet liner is slippery, try it with a nice coating of algae
Again welcome to the forum, thanks for the HA-HAs, if I can be of any help, please don't hesitate to ask! - Waste