1/2. Code review can always have comments, but you're right that the nature of those comments will change with seniority.
Also, have seen some principal engineers not sharing their pull requests as they are quite high in seniority and merging it directly.
This is a red flag to me - Just because you're very senior doesn't mean you're immune to feedback. Principal engineers make mistakes too, and more junior engineers can definitely leave salient feedback on PRs of engineers more senior than them. Staff and senior are below principal as well - I'm sure engineers of these level can chime in with something useful now and again on a principal engineer's PR.
I would bring this up with your manager and see what they think. Pitch opening up these PRs as a learning experience for the team - I have found that simply reading the context section + test plan of a code review by a very senior engineer can be an educational, "level-up" experience for a more junior engineer (a lot of my mentees learned from me that way).