Downvotes may have some merit, I haven't thought enough about it. On the surface I like downvotes as being somewhat less prone to vote buying, self-voting, etc. We see that clearly with vote payouts and some of the same issues apply to witnesses. That being said, overall there are various angles that all need to be considered in evaluating a voting system (fairness, representation, abuse- and corruption-resistance, security, etc.) and it isn't a trivial effort.

If by dead man switches you mean some sort of vote expiration/decay, then I agree with it, though the devil is in the details. IMO the most clearly supportable version would have votes from completely idle accounts decay or just removed after some period of time, say two years. Other versions are plausible too, but the tradeoffs are less clear.