I think you described the landscape well -- mainly if you do the wrong thing you'll be fried.
So, with that in mind, I c-a-r-e-f-u-l-l-y offer these thoughts -- with the usual disclaimers.

Assumption #1. People will want to get started playing the game as quickly as possible on their own computers. To slow down that process in any way will cause ill feelings discussed in the article.
Assumption #2. Multi-player is going to be THE key feature that attracts people to this game (yes, I know that it is sooo much more - but MP will be the first attraction).
Assumption #3. As long as people can get started with the game on the local computers AND at least get started with multiplayer -- with the least possible interference the better.
Assumption #4. People will welcome the opportunity to \"step up\" into a higher level of competition for those who have completed \"base requirements.\"
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Okay, now my idea ...
- Let people start the game (off and online) without
registration. Let them progress to level X.
- But to gain \"special achievement\" rewards (such as expanded scenarios, multiplayer levels, command levels, etc), require that they log in to register their progress online. Have the unlock delivered online.
You will have to be careful how you describe the unlocks. But, just for example, you could have multi-player \"command levels\" be monitored in this way. Do you really want a \"newbie\" commanding a whole army online? They would only get frustrated and leave! Maybe they could command the whole army but there would be some special functions that they couldn't access until they've showed some mastery of it.
Okay, maybe my example isn't that good -- but I think the idea could work.
Cheers!