Indeed, the game scales the levels of the opponent and nothing else. All bonuses from cards stay the same. Someone who has a lvl100 character most certainly boosted their team to hell and back. Here's a comparison of my Lucia with and without stat bonuses. They say that you can play this no matter your level, but that is absolute bullcrap. Newbies will get mauled at least until people get put into proper brackets.
You can purchase materials with the medals you win. The Yoshino card is a reward for climbing to the fourth class (Why would you want a suicide attack though). You're only promoted to a higher class at the end of the season (Notice it says season 1 on the event page). This game mode is obviously not going away as quickly as previous events.
This looks like a really good development for the game as an alternative to the regular stories that you can just auto through and the challenge quests that rape you hard and give shit rewards.