Have you fully consider the Designer's side of this? I feel that to be successful and bridge the gaps between those two mental models you also need to onboard the designers so they can help with generating the right type of mockup (or at least the kind that will help a developer using Pagedraw get the closest to a 1 to 1 code export).

The reason why I ask is that I tried importing several semi-complex but not uncommon (structure wise) files and I got an error saying that I should make sure that the file is correctly formed.

That right away is bad UX. If the designers need to reformat or change the behavior of their files, then they need to be part of the user journey. Right now there's nothing of that as far as I can tell.