I have noticed an issue/exploit in the Powerplay mechanics of the game. Players can use solo/private mode to undermine expansion and fortification efforts without the victim factions being able to retaliate. This leads to the attacking faction being able to camp nav beacons and combat zones and farm merits without any sort of real opposition. Please make it to where Powerplay actions can only be performed in open play as Powerplay is a multiplayer mechanic and should be treated as such.
Ah, yes, the overly simplistic definition of the problem in an attempt to generate more targets.
Things not considered in the summary:
1. Timezones - Unless you have 24 hour coverage of the specific system by your players, any player that is not in at the same frame as you is as "invisible" as someone in a different mode.
2. Instancing - You may just not see them as instancing is not guaranteed.
3. Trade - You can influence Power Play without combat.
4. Merits - If the only objective of your opponent is to "farm merits", they can simply go somewhere that you are not. Since Power Play is not a real time activity, you would have no indicator that they are there unless all you do is monitor the Power Play screen over time, which would effectively take you out of play anyway.
None of those elements involve mode, yet all of them would avoid your stated reasons for forcing people into Open.