If more than 1 collaborator adds a new behavior, effect, object type, family, layout, event sheet, script, or timeline, it modifies the .c3proj and creates a conflict.
This should not create a conflict as long as separate parts of the file were modified. It should only be a conflict if two people both try to commit changes at the same time that both affect the same line (e.g. both adding an object to the same folder).
That really is just how source control works - I suppose Construct could load things some other way, but that doesn't prevent conflicts. You'll still get conflicts if two people modify the same image, edit the same comment, change the same instance variable, etc. etc. The best approach to avoid conflicts is to assign clear roles on the team - for example we have clearly assigned code areas for working on Construct itself, so we rarely have to modify the same thing and rarely ever get conflicts.
Again, not the expert here, but sequential UIDs might not be the safest approach.
It's true sequential UIDs don't work well for merging, and it's been a problem before. However I think there's a smarter way to solve this than just adding a whole new kind of UID: in r348 there's a new project property you can change that assigns all UIDs as random numbers with at least 6 digits. Therefore if two people add new objects, the chance they are assigned the same UID is negligible. I think if you set your project in to this mode it should avoid this type of problem happening.