There's "short" Golden Week (May 3-5) and the longer Golden Week (April 29 - May 5). Plus add weekend days and you can go from April 27th to May 6th. That doesn't mean everyone will have all that time off, but plenty of students do, and so do tourists from other countries (like South Korea and China, which both have a similar holiday structure, though not quite as long as Japan's). If you haven't already looked for accommodation for that time period, I would suggest you do it now.
As for your itinerary, I agree with the others that it's too rushed, and there is no need to go to Osaka or Okinawa (the main island has mediocre beaches and early April is not the best time for beaches anyway). I would take those 4 days you have planned for Osaka/Okinawa and add them to Kyoto or Tokyo or day trips from either (for example, Koya-san from Kyoto - also good as an overnight). You seem to be spending a lot of nights in cities that you don't explore much at all.
Matsumoto is boring (I've been there several times) - it certainly does not have the variety of restaurants one would see in bigger places. My friend and I had a hard time finding a place to eat there (she's vegetarian) and we wound up at a chain pasta/pizza place which was alright, although we had hoped for something better. However, as a place to start your trip to Kurobe, it's fine. Not every night has to be a big night for dinner and/or drinks.
