1. The arena x1 is ok for battling since it is easier to target and think of what will come to deal with. The arena speed can be fixed to x2 by the the agreement of both sides ( just a suggestion) but it wouldn't happen. Why? Because in the Chinese version, the arena system will be the same, they just add another mode that lets you test your team without losing any points.
3. Nope, Fire main in the future will be stronger than this since his 10 combo triggered chase can cause Ignition to 2 random bodies. For now, of course, only deal fire damage . In the CN version, each main character will have one more selection for each row, which means they will be added 1 more column and if you are not fond up using the sleep standard attack, you c*e a standard attack that can attack up to 4 bodies and have a probability to cause Ignition to all bodies hit. And he will have some more supporting skills, etc. He c*so cause blindness with his chase, too ( this will be in the same row as the celestial prison and the 10 combo). In conclusion, he will get a small buff in the future, not anything near to get nerfed.
For the sleep attack: try to spam clones, puppets to avoid the sleep standard attack, he only attacks anything in front, in sight and near or in the same lane as him. Or you can bring someone to disperse the debuffs, that just some of my ways to deal with it, there will be cons and ways for the sleep attack to slip into your team but just do as much as you can to prevent that.