The issue you describe was a bug in AUCTeX which was fixed with this change. This change loads hyperref.el when you request the beamer class and are asked for class options, making LaTeX-hyperref-package-options-list available before the style hook runs.
I would use
#+ATTR_LATEX: :width 10.6\textwidth
This is described here: https://orgmode.org/manual/Images-in-LaTeX-export.html
For specifying image ‘:width’, ‘:height’, and other ‘:options’, use this
#+ATTR_LATEX: :width 5cm :options angle=90
This is related to the number of levels exported as headlines, which is controlled by org-export-headline-levels. Which value does this variable have?
The template you pointed to includes #+OPTIONS: H:3, are you sure you didn't alter that value?