So, the counterfactual as it stands right now is that we're giving somebody additional mental powers through high-speed access to software hooked directly into the brain.

Um, no. That's part of the issue -- we're not giving her access to additional mental powers. We're giving her easy, fast, and convenient access to some information tools. Her mental powers remain the same -- if her working memory is limited, it remains limited. Being able to look up things in a second does not imply a large working memory. If she gets confused with longish logical chains, direct access to Excel isn't going to help. Etc., etc.

She is never going to make an error in arithmetic or algebra

Oh, but she will. Go talk to, say, accountants -- people who professionally use Excel and have been doing it for a while. Ask them if they ever make an arithmetic error :-)

Excel permits programming, so the scenario does seem to allow us to give her some Watson-like powers in addition to everything else.

Well, Excel includes VB which is Turing-complete. So you could treat Excel as a general-purpose computing environment and provide her with an narrow AI which, basically, solves the test for her. But I don't think that's what we are talking about :-/