Welcome back. Here it goes:
(1) Is Mara the most powerful dewa, the fact that he is the king of the highest dewa realm?
Mara is one of the 2 most powerful devas in the highest sensual realm. The righteous king is Vasavatti Deva, and the other is this Mara Devaputta.
(2) Why is Mara not supportive of those practising dhamma? (eg. in a story of Arahant Uppa Gupta, who existed 300 years after Buddha's passing away)
Mara was jealous of those who could attain jhana as they would be reborn higher than him. Mara has great power that he can enjoy objects created by others for him. The story goes that Upagotta had subdued Mara and he had promised not to disturb any follower of the Buddha ever again.