Dear Julius.
Welcome to NMB! Your sword is indeed a wakizashi, the pictures of the nakago, tang, indicate that it has been shortened at some time in it's life. The mounts are complete, though I cannot see the kashira clearly and that may be a replacement put together when the hilt was re wrapped and that by someone who did not know what they were doing. In the sword's present condition it is going to be hard to say much more about it with any degree of certainty except that it is an original Japanese sword, it will be at least as early as the 19th century though the shortening may suggest earlier. The tsuba, guard, may be an early one.
You have resisted the urge to clean anything, stick with that idea, just a little light oil on the blade while you work out what to do. If it were mine I would probably arrange to have the hilt re bound by an expert and keep it the way it is now. As to value, well not a great deal. In the UK a few hundred pounds, probably the same in the US. As a family piece brought back by your grandfather, of course it's a treasure.
Hope this helps, much more to come from others I am sure.
All the best.