Hi Ginger,
Here's my slant on the scenario and battle to answer your questions. (Having computer trouble at the moment but hopefully it'll stay on for a bit):
1. No. In truth I was expecting Tyranids. But the set up insures some balance in that you don't know whether you'll be the attacker or defender.
2. The SM had 3 Scouts, a Predator, and a Warhound. The warhound wasn't hidden as per the rules.
3. The 'Hidden Unit" idea was in the original E40K Battle Book. Keeps the attacker on their toes. Whether to defend forward or back is a critical decision.
4. Again, from the E40K Battles Book. They could come on anywhere except the side the Eldar arrived, provided they came on at all. The SM's didn't come on in the Eldar deployment area. Eldar were off board and had none, though the Marines couldn't deploy their initial force on the furtherist 20cm of the table.
5. If you'll recheck 'set-up' you'll see both players put down 3 markers on the Defenders 1/2 of the table, THEN roll 2D6 to determine the attacker. Keeps everyone honest. I'd already set them up, but Greg was free to switch around how he liked.
Observations:
1. As Greg said, our Friday games are a lot more structured and played to Tournament rules. Still, I come from historical games and enjoy refighting a battle or doing a scenario. Both are great fun and we don't get locked into doing the same thing.
2. Objectives were the blue or green (3 each) pegs. 3 placed by each player before sides were determined.
3. We will, but not right away. I have about a dozen scenarios to go through yet. Maybe around the end of July.

With the aid of Greg's photos, it'll be easy to reconstruct the battlefield.
4. Other factors apply. Usually, if an army is going to make it to the objectives, 6 turns should be enough (as per the E40K Battles Book). 6 turns are also about all the time we have for a game (3.5 to 4 hours).
Scenarios can be much more challanging than the usual "meeting engagement/Tournament game", but can also go belly up more easily as well. I just try to make things as even as possible given the victory conditions, and let my opponent choose the side. Like a kid cuting a pie, it's amazing how fair you can make things if you know you're getting the last slice
