This will be even bigger issue when players can participate with just a lvl30 character in PvP (you could even skip leveling with jump potions if you wanted to). They will just make an alt to prevent their team from losing 2 many points in premade.

I am too in favor of restricted access to premade pvp, it worked in other games. Timed windows on a day where you can play it.

The rating system needs to be reworked in premade pvp, only because a group is unranked shouldn't mean you will lose up to 50 points against them as ranked. These rating spikes should be way smaller, vice versa you should gain more than 1 point for unranked teams. It would just even things out. There needs to be encouragement that you don't want to play with an alt or that the risk playing with the main character is suitable.

In some other games, ranking systems work like that a team has to prove themselves first in a number of matches, if they are glorious they rank up and get "real" points for any further matches. We got unranked and bronze, if unranked players would be considered out of the equation for rating until they reach bronze, you would minimize the benefits of playing an alt in premade aswell.