Some simple observations.
To perform such a QALY estimate you need
A credible model for predicting the consequences of possible responses
An estimate of how likely your advocacy is to effect policy
1 is something you need to even know what the best response it (and I’m currently not sure whether you have it).
2 sounds like something that should have been researched by many people by now, but I’m far from an expert so no specific suggestions.
I think downvoting as disagreement is terrible.
First, promoting content based on majority agreement is a great way to build an echo chamber. We should promote content which is high-quality (well written, well argumented, thought-provoking, contains novel insights, provides a balanced perspective etc.). Hearing repetitions of what you already believe just amplifies your confirmation bias. I want to learn something new.
Second, downvoting creates a strong negative incentive against posting. Silencing people you disagree with is also a great way to build an echo chamber.
Third, downvoting based on disagreement creates a battle atmosphere. Instead of a platform for rational, well-meaning debate we risk turning into a scuffle between factions with different ideologies.
All in all I think the rules for downvoting posts should be slightly more lax than for downvoting comments. Downvoting a low-quality post is acceptable (but be very cautious before deciding something you disagree with is “low-quality”). Downvoting a comment is only acceptable when the comment is not in good faith (spam, trolling, flaming etc.). I think this is essential to maintain a healthy amicable atmosphere.