Arcade was free if you purchased the Seasons in the past. As of now, it seems there are no plans for this mode (aside from season 1) to be free.
For SemiEvos, sadly no. They will level up if you use them with no reverse. You could prolong them if you are part of a Guild that doesn’t have the Character Xp boost on.
In addition, losing with a SemiEvo also prolongs them, the latter method defeating the purpose of SemiEvos in the first place in my opinion. I would try to look for cheap SemiEvos if you are insistent on having them as some players I see are successful without them.