The stamina isn’t a points based system, it’s just to check for inactivity. Having a negative stamina will affect you if you drink a potion to gain more, but you’ll start the next day with a normal amount of stamina. As for the Reaper stamina penalty, you can negate it by completing a quest to recieve the item “Soul Candy” but you first need to get the allies TicTac (from Gen Reaper missions) and Robogirl (from Tai Reaper missions) to unlock the quest. There are similar penalties for a few other parts of the game but they also have ways to safeguard against the stamina loss.

As for how the sparring and arena work, it’ll pick a difficulty and have each character roll as normal (it counts your levels, range, strength) and whoever gets most successes wins. Arena *usually* pairs you up with other players of the same season and rank, but there is a percentage chance to face anyone in the game.