Hi Mac,
What I would do with the sword were I in your shoes.
Do not clean the tang; do nothing whatsoever to this part of the blade.
Corrosion/gunk on loose metal pieces (tsuba, seppa, whatever) can be cleaned with warm water and a toothbrush.
If the pin (mekugi) is loose or weak I would replace it with a new one, which I would whittle from a bamboo chopstick. The mekugi is very important as it locks the blade in the handle and doesn't allow it to fall loose and shatter its point in the scabbard bottom or on the floor.
Coat the non tang portion of the blade with a very light coating of machine oil. Get an uchiko puff/ball and use it to remove a lot of the surface corrosion on the blade. Here is a link to a care & etiquette site that explains how: http://www.nbthk-ab.org/Etiquette.htm
I wouldn't have the blade polished (goes without saying that I wouldn't try to polish it myself; that would be insanity) because the expense isn't justified by the value and the corrosion isn't bad enough to bother with. If you wanted to spend some money to make this more like what it was when made, you could have the handle rewrapped and the scabbard reglued. You could look into a new, lacquered scabbard but it looks like the uncoated scabbard is the original condition.
Resist all temptation to wing it (come up with your own formula for restoration). Chemical rust removers, abrasive pads, Elmer's Glue All and pretty much everything else you can think of will only dig the hole deeper.
Grey