The last direct quote I saw from Cameron had him say he was writing the draft, hadn't heard anythng back from Scott about it (and never uktimately did), then mentioned it to someoneat Fox, who casually mentioned they were doing an AVP concept film: They had been naware Scott/Cameron were planning a fifth movie. He said it took his inspiration away or someting (but that, after seein it, it wasn't the Godzilla-alike toy adver he thought it would be, after all).
From that, my reading of the situation is that Cameron was ultimately the only entity involved who was actively trying to do something about getting it made. My guess is that Fox may well have gone with their concept instead of Anderson's, if they had known about it in advance. Weaver has mentioned having diners with Scott where the ideaof a fifth movie was brought up several times, but nothing about Cameron. So, I'm guessing it was just the two of them whe were making plans.
As for Cameron's writing skills, he's churned some excellent stuff out, script-wise, but he's always been an avid reader of science-fiction and my guess is that the better quality stuff rubbed off on him.