Again, something that WDG have looked at multiple times in the past while I was an admin/leader and generally Lefty's #1 solution was where we ended up back at.
The problem with #4 is that it interferes with manual alltalk settings - so if you turn on alltalk manually the plugin will switch it off again so it became more of nuisance
#3 - yes, there are preferences plugins but again, it means that. Normally with those sorts of things you want it to be decided at the start and stay like that until the end of the map
Yes - it should be possible to modify the sourcemod permissions to allow anyone to start an alltalk vote - not sure about doing the rtv style where a minimum number of votes is needed to start the vote
The chances of there being a time where there are 0 admins on the server or 0 admins online (remember - most admins can turn on alltalk without playing using hlsw).