I think full-blown collaboration is a really difficult feature to use in practice. This is my example of how tricky it can be for users:
Alice and Bob are both editing the same project
Alice adds a Sprite
Bob adds an event for that Sprite
Alice presses undo
What happens now? Does the carpet get yanked out under Bob and some of his work just vanishes? Or is Alice simply unable to undo - so a key feature of the editor is disabled? I'm not convinced there is a good answer to situations like this, and it seems like collaboration would quickly break down in to a nightmare of either being unable to change what you want, or trampling over other people's work.
There's already a great way to organise collaborated projects - source control does it well, and you can already use that.
There is a good answer, it's called a pop up message...man that's basic UX stuff that many browser tools
already figured out...
The problems you are listing are really but really easy to solve, a team working on a project are in constant communication, there can even be an implemented chat and all, stuff like "not being able to work" will never happens, there are also soooo many softwares that have collaboration features in real time even softwares for video editing.
I will suggest you Ashley to do some research firts instead of saying no from the start, since it's not something impossible to do and that many softwares do already, all the problems you are listing are really but reaaaaally easy to solve for an UX expert.