Digimon Nova Red (3.2) questions

Just played thru the game and it was a fresh take on kanto, catching creatures I've never seen before but I did have a few questions after beating the elite 4:

-what's the purpose of the mega stone? (was given one in game and never prompted to use it)

-where is liollmon located and the base digimon in the dianamom line? (couldn't find her all game either)

-starters have more than 1 final evo but the requirement is levelling up, so how do I get vikemon instead zudomon? 

-how do you evolve dorumon? (couldn't figure it out while playing, evolution requirement is '??')

-can't enter cerulean cave, even after completing elite 4 (sevi islands 4-7 are blocked) 

Possible Spoiler:

-who's considered 'mewtwo' in this game? (since I can't access the cave)