How can you say it's RTW as it should have been? Egyptian chariots in the 3rd Century B.C. shouldn't have been. Some of us are still waiting to play a decent RTW campaign, and it shouldn't require mods to do it.
No, the expansion does not fix the Egyptian chariots and shrieking women, etc., in R:TW. If I understood Froggie correctly, though, she was asking if the Barbarians campaign was as good and challenging as R:TW should have been. At least that's how I understood her question.

Clearly I agree with you about the silliness of Egyptian chariots in the 3rd Century, and can find quotes on other threads where I've said so many times, Puzz3D. Let me put it this way: If Barbarian Invasion had been the original game and R:TW the expansion, it seems there would have been no Egyptian chariots.

To be perfectly explicit:

Is the Barbarian Invasion campaign better, more challenging and plausible than the R:TW campaign?

Yes, much.

Are there considerable improvements to the tactical game that will help make the original R:TW game much better?

Yes. There have already been numerous examples, and I'm about to post another one. See my upcoming post about armor and archery.

Does the expansion solve the "hard-wired" in-the-design problems, such as Egyptian chariots, that spoil the fun of the original campaign for many who have some sense of history?

No.

Is Barbarian Invasion a seperate game -- more of an improved full-blown sequel than an expansion with a bonus game, ala Viking Invasion?

Yes.

Can you play the Barbarian Invasion campaign with a knowledege of history without feeling irritated by some of the liberties taken?

No.

Can you play the Barbarian Invasion campaign with a knowledge of history without feeling like you just landed in Oz?

Yes.