I'd say that for most people it wouldn't really matter in what order you play the scenarios. Plus if you're going to play on the scenario basis you would want to change up who you're playing as each empire can be quite a different experience. It wouldn't really make sense to "pick up where you left off" except in the campaign context, because otherwise you would be throwing away all the accomplishments you achieved.
You're doing one of the scenarios rather than the whole campaign, right? Probably 4-5 hours, depending on familiarity with the rules. You can probably shave about 45 minutes off that if you choose the shorter 1386 scenario.
having never played:
- do you play the scenarios "in historical order"? (does it make any diff? maybe pick-up where you left off next time?)
- by chance are approx. play times listed with any of the scenarios?
Yes, in this one every player adds a proportional amount of time.
- and would playing with less players also lessen the time?
It's not as bad as in other games because here you spend more time in this game trying to cope with the game system than you do with other players.
- is a 2-player game worth the investment in time?