I understand that you don’t get the error messages for materials not found. This might mean that you are a tad closer to a solution. It still won’t find some materials without adding their directory specifically, but that may be a macOS quirk.
At first, 4.26 wouldn’t open a copy of my 4.25.4 project at all. It would crash at 95%. I got it to open by creating a new project in 4.26.0, gutting it of everything and then migrating my whole project (from the top content folder) to that new project.
With the new project that I have created for testing, I can now create a widget and a stereo layer in it and have it to launch correctly in 4.26.0. What it wouldn’t want to do earlier (crash upon opening) was to to deal with an imported widget that had bindings and scripts attached.
It just tells me that it can work, but it’s an ordeal.