Niggles: Question. If i start a saved game with a given mod at a certain version number, do i need to restart the game again from scratch if i want to upgrade that given mod (or any mod) ?
For most mods, this won't be a problem ever.
The more complex mods, may have a specific way to upgrade (or remove) that you need to follow. So whenever you update mods, check their description page for installation / update instructions and you're on the safe side.
Niggles: How long is the actual main quest line?
I'd say about 10-20 hours, depending on how much you have to walk or can fast-travel.
Elmofongo: Its a wonder why no one made a Cyrodiil, Shivering Isle, and Skyrim mod using Morrowind's engine and rules.
With Morrowinds rules, ok. With it's engine? Why should anyone want that?
Elmofongo: I especially think a Skyrim mod using Morrowind engine can be done because of the same landscape Solstheim in Morrowind and Skyrim looks like so modders can just use Solstheim resources to make a Skyrim mod.
Yes they could use Morrowinds Solstheim assets to create the rest of Skyrim. The question is: why? Absolutely nothing of importance happens in Skyrim at that time and the next big change in the world as a whole, is the devastation of Vvardenfell and the destruction of Vivec city.