This simplest and most logical solution is for merits to be awarded in all game modes and have only open play effect control points.
In principle, I'm not opposed to weights to account for the de facto increased difficulty for some activities, in some areas, in Open. However, in practice, it's trivially easy to select 'Open' and turn it into 'Solo'.
For this to change there would need to be relay servers to bypass IP filters (a firewall that only allows IPv4 connections from Frontier's servers prevents any peers from connecting) and significant changes to the block system, which aren't going to happen. Failing that, some system to track actual encounters would be required, which could easily be gamed. Incentivizing Open can't really work if there is no way to even enforce the fundamentals of the mode.
The simplest and most logical solution is to remove open mode, as it's only open players who constantly moan about what other players are doing in the game.
No more open mode = no more reasons for them to moan about what I'm doing in my PG (as they'll be in their own PvP PG)
Since this is a multiplayer only game, irrespective of mode, I have to see the mess Frontier have allowed us to make of the setting, irrespective of mode.
What you do in your PG affects my game, always has. I don't need to see you make those contributions to experience their effects.