Can your party only have one active quest at a time ?
How can you know which quest(s) is active ?
Are most quests repeatable ?
Does the quest order matter much since most encounters seems to scale with the party ?

Quests are tracked by which items you have in your possessions. But the Class Quests are restricted by class. IIRC Class Quests are not repeatable, while those that require an item generally are.
The game is not strictly level scaled; it's a combo of level scaling and area scaling, as well as fixed encounters, so some quest will definitely by harder than others.

Only one active quest at a time. The quests are repeatable (not sure if they're randomized or not).

There is a quest removal elixir in one of the castles.

Note that the Class Quests are not technically quests for this purpose. I would expect them to be repeatable, provided that you either have nobody who's completed the quest or at least one character who hasn't completed the quest in your party. (This, of course, requires that you change party members.)

Also, many events that give you items will only trigger if you don't already have that item in your inventory. (Note that you can work around this by giving the item to a character not in your main party; there's also the trick of giving the item to a hireling and dismissing them from the party while they're carrying the item.)

Edit: Also, I believe the only thing that scales with your level is the number of enemies per encounter, not the strength of them. One tip for those big combats: Turning off sound will greatly speed up combat, then Power Shield + Ctrl-A will let you win most encounters in a reasonable amount o time.

No. You can have as many "quests" as you want at a time. The restriction was there only in MM1 and only for the Lords' quests (you could only have one quest from one Lord at a time). All other quests, and all MM2 quests, have no restrictions.

There are some "events" in MM2 which are tracked in the save file on the PC version at least. The ones I'm thinking of are:
* The "Spirit of Corak" that appears after you take your first step forwards out of the inn
* The Pegasus quest
* Lloyd's assistant in one of the castles only appears once

Probably many more I can't think of as well.