What if comments and engagement were another way to inch a post to the top of the list so it was more likely to be viewed, and therefore, voted upon?

I agree that additional metrics would help boost posts with interactions.

A) would this be too easy to game?

My concern are the bots. It's easy to spam a post with lots of artificial comments.

B) Is this already happening cuz I don't think it is

No, a lot of interaction in comments does not help boost the visibility of a post

C) does bringing more proof-of-work users syphon away the HBD value from investors, or does it create an opportunity for more?

It's difficult to say. If investors simply desire ROI, they can purchase HBD and put it in savings and watch it grow effortlessly. If investors want INFLUENCE, they could purchase a ton of HIVE and power it up and upvote and downvote to their own pleasure. It takes a lot of effort to grow at 20% APR, so anyone investing their time and effort is doing it for other reasons than just ROI.