1. Rapidly diminishing returns for killing the same Cmdr repeatedly in a given day/cycle.
I think this is probably the way to go, although this can be of course circumvented by a dedicated group with several alts that for instance, went in a circle with each person killing everyone else's alt once.
2. Missions require killing Cmdrs from a specific finite list of enemy pledges - your alt/friend may not be on that list even if pledged to the enemy power. If everyone goes and has a big fight then points will be scored, but players won't control which kills count.
This would be very troublesome. As I'm sure many PvP people are aware, the issue of instancing is so problematic that instancing with one person on a list, let alone a specific commander could be very troublesome, especially given things such as timezones, logging in, and other potential irl complications that would make completing something like this be impossible.
3. Tie the weight of the PvP points to a PvE task - so I have to do a balanced proportion of both in the same interval to get a good score.
I think this could be promising. I would prefer to not be forced to PvE to make my contributions stick though.
4. PvP points will only be scored by players in open play who have no blocks against players currently in-system.
Definitely agree with this, though unfortunately even with this kind of change, haulers are still advantaged unless there is a score decrease similarly applying to haulers/other kinds of gameplay loops with in-system blocks.
5. There will be more efficient ways of scoring PP points than PvP kill-farming. I.e. PvP points are a small bonus that partially compensates those that dutifully switch to a PvP role to protect the PvE point-scorers.
All the same though, securing a legitimate PvP kill can be substantially harder/more time+person intensive than most other activities, so it would be nice if the rewards were big for that too.