PvP is one of the only ways my CMDR can influence PP as he will never pledge to any power.
You can also influence Powerplay via BGS manipulation, no pledging required.
No PvP required either.
Just like any blockade. Even during the peak of the Battle of the Atlantic, during the golden age of the U-boat, something north of 80% of allied shipping made it through.
You don't need to stop everyone, just enough to tip the scales.
The thing of the matter is, unlike those U-boat captains of old, you have to contend with the issues of an online game, as opposed to those of real life.
In real life, both sides are in the real world 24/7/365, and can never physically leave it.
In the case of online games, only one side is required to do this. The other side can log in any time they choose, get the job done, and then log back out before the first side can respond. They can also use alt accounts to teleport all over the galaxy if they so desire. Elite: Dangerous has the additional wrinkle that the first side probably won't be able to see the other if they're physically separated in the real world, multiple platforms that don't connect with each other, or just aren't instanced with anyone else thanks to the vagaries of the matchmaking server.
The way matchmaking in this game "works," you could be on patrol for hours, and you would never see the dozens of players flying past you in different instances, due to them being half a world away, playing on a different platform, have an ISP that blocks peer-to-peer ports, an overly aggressive firewall, or are behind an ancient piece of telecom tech that shoots their latency through the roof, which the local phone company has no intention of replacing because they're only one of three houses connected to it. (The last describes my situation with the local phone company, which is why I have to go with Comcast.)
Blockading works if the amount of effort required to maintain the blockade is less than the amount of effort required to get past it. In video games in general, and in this game in particular, the opposite is true, by a massive amount.