Apply Master to all slides?

I would be happy to help you, but allow me to say that your idea for a 'debugging' mode is not that good for several reasons:

1. The original design of all content slides will be messed up. I suppose you are using a custom theme and have tems o the main master slide and dauhhter master content slides.
2. Blank master is a 'reserved' master slide which is used for software simulation slides and PPT import slides.
3. Blank master slide can not be applied to quiz slides nor score slide.

If you expkaied what you want in 'debug' mode. When I am debugging a file I want to have these features:

1. quick and easy navigation because I often have custom navigation like going to next slide..
2. tracking of some system variables and user varaibles

I realize 1 by adding temproraily the TOC (overlay) with full navigation capabilities and sometimes also group slides for that puppose Turning it off  is very easy an uick.

For 2: Create one or more shapes with the variables, put them on the first slide (where I need it) and time them for the rest of the project. After debugging I drag the shape(s) in the scratch area?

Several users will complain (again) that I a lecturing instead of answering the question but thought it important in this use case.

Answer: you can obbly switch the master slide for a bunch  of slides if they originallused the same master slide. If they used different master slides, it is not possible.