Ok, so right now the Inset command is expecting to have a border area, it isn't possible right now to reduce the border size to 0 like I think you're asking about.

It seems to work ok over here if I leave your pieces all joined, do you have a version with it joined up that won't inset for you?

What you really want here is a way to cut a groove in an object just directly using the 2D profile curve for the cutting. I definitely want to pursue some more stuff related to that in the future but probably it will be initially focused on cutting indentations into a solid using a 2D profile curve where you'll be able to do "depth limited" cuts. Right now you can use 2D curves for doing a boolean but it cuts all the way through the object, not to some limited depth and that's what I would like to add. It would also be good to add some kind of similar groove cutting mechanism where it would be based off of cutting a solid with the profile curve rather than one based on face selections like how the current inset command works. I'm not exactly sure where that would go though, like whether it could go into the boolean commands like a depth limited indentation cut or whether it needs a separate command.

Anyway the current one being based on face selections does not allow for border to go down to zero in width - that would not work work in many other cases like for example if you tried to do a zero border width indentation on one face of a box that would be weird. That's basically why it does not try to do it currently.

- Michael