Can Aragorn from The Flame of the West (with the fellowship sphere) can be played in a mono (single player) game? Being used all the time by the same player?

If you play in the appropriate scenario you have to keep it constantly if you are the only player. Outside of those 3 quests you can't play this Aragorn, no matter how many players there is.

So all fellowship cards can be used only on specific scenarios? Not in all?

Fellowship cards are only mean to be used in the saga. Fellowship heroes (with 0 threat) can only be placed in play by scenario rules. Sometime it is a Frodo and sometime it is Aragorn.
Fellowship player cards can be put in your deck every time you play a saga scenario but only when playing saga.

Thanks!