Engagement ...

• Isn't it that 'famous' users are always receiving a huge number of automated upvotes anyway, independently of the quality of their content? Would there be a way to count only manual, non-automated upvotes (of course excluding bid bot votes, too)? Probably not so easy, I guess ...

• Concerning the comments: couldn't the length of the comments play a role as well (word count)? Even better would be to analyse the structure of comments anyhow (otherwise abusing users would start to create automated long comments).

The 6.5 days is there because the last 12 hours before post’s payout only downvotes are allowed by the blockchain due to security reasons.

That's wrong! Since HF 20 there is a cool-down period instead:

Velocity will address this potential scenario by modifying the lock-out to a cool-down. After Velocity, upvotes and downvotes will both be allowed during the last 12 hours of the payout period, but their strength (for the same amount of voting power) will decrease linearly from 100% to 0% over that 12-hour period.