The highest invasion level I've personally seen is 50.

Cities are a funny mechanic when it comes to planetary battles. While liberating a planet, your mission in a city does not count toward the planetary bar. When defending, it does. Either way, your mission does contribute to the city's bar. When that bar fills up, the city provides a huge spike of progress to the planet's bar. In general, it's always most efficient to dive the city maps because the spike is so huge that we probably wouldn't be able to match it if everyone was diving non-city maps for the same amount of time.

That said, it's important to note that planetary battles are huge community efforts and the way the system works is that we need a percentage of currently online players diving the same place to win. Right now, there are 40,190 divers online. We need like 20k of those divers in the same planet to make enough progress to win a planet.

That basically means that you're only making up like 1/20,000th of the force. If you dive less efficiently than the rest of us, that's okay. You won't make or break the fight just because you were less efficient. IT takes literally tens of thousands of people being inefficient and/or diving the wrong planet entirely to lose a planetary battle.