On #2, the scenarios, or rather the main campaign, is an interesting challenge, and quite different from what you'd find in a normal game. You spend most of the campaign fighting against an opponent with vastly superior technology, which requires a very different approach. A couple of the scenarios in particular are especially tough, and very satisfying to win.
On #3, a larger galaxy and more opponents does not necessarily make for a more difficult game. The human player seems to benefit the most from very large maps; the AI is surprisingly good at short-range colonization (they routinely beat me to nearby worlds), but not so good at long range colonization. The Medium map seems to be about perfect for the AI; Large is just a bit big (I can usually find uncolonized corners) and Huge is a human colonizer's dream.
I think fewer AI opponents may actually be more difficult. When there are numerous AI opponents, they balance themselves out for the first part of the game, until one of them gets an edge, but usually by then I've outgrown the leader by a substantial margin. That doesn't happen when there are relatively few opponents.
In short, the Huge/Nine opponents setup may not actually be the most difficult (I hope whoever's doing the coding for the Metaverse is listening); the usual "expand like mad and play the AI off against each other" strategy works like a charm for that format. If you really want a difficult game, I'd suggest going with Medium size against a lone Drengin opponent, or Large against a Drengin opponent and a Yor opponent, playing a Good race, of course. If you want even more of a challenge, try playing as a race that starts in the corner (like the Thalan) against a race that starts in the middle (like the Terran or the Drengin). I haven't tried any of those setups, but then I'm not a glutton for punishment
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