Mobile-first responsive order summary component

Question about html structure:

1. Should two last elements (proceed & cancel buttons) in my card component be grouped into one cta section? It seems more logical to me but I have some doubts. General questions:
2. Which css naming conventions would you suggest when card layout becomes really nested (e.g. container1>container2>container3)? I have read about gcp and bem, but the last one seems pretty hellish when you have to name nested children with parent prefix and it goes like 3-4 levels upwards.
3. How to learn to be more precise while figuring out margins, paddings, font-sizes/weights if you don't have design assets?
4. What tools/platforms could you recommend for cross-browser testing? I have found browsershots but it doesn't seem to work with netlify deployed sites.