I've sailed on Princess many many times and have never encountered problems like this. We've also had alot of clients doing that same ship and itinerary without problems. They do say the Island Princess is not quite up to the standards of Princess' newer ships, but overall they've reported it to be very nice.
Sorry to hear you encountered some problems, but one cruise on one ship is usually not indicative of other ships or the cruise line as a whole. We've been on one ship three times and each of those provided us an entirely different experience - things change, especially the crew, which can have an impact on the overall experience.
But as was stated, the cruise line has the right to change/cancel ports based on the most current and ever-changing circumstances. They'll never put their ships, crew, or passengers in harm's way.
As for saving seats, I agree this is a problem, but for the most part, this is a problem on every ship on every cruise line. Some just deal with it better than others.
But the cruise line will not refund you any money. The only refund you should have gotten was an onboard credit in the amount of the port fees for those ports missed. They may offer a future cruise credit or onboard credit for another cruise, but they will not refund any cash, so you will be disappointed in their response.
But the good thing about cruising is that there is something out there for everyone, so perhaps another cruise line will better serve your needs. And that the fun part, finding a cruise that's best for you.