December is when tourism starts picking up. Its the beginning of the dry season, easier to get around, and its cold in america/europe so everyone wants to go where its warm. Late November early december is a good time to come because the rainy season is ending so everything is still super green and lush, but its not all muddy and wet all the time.
Yes, they cut the power off at pre-arranged times. Its all from diesel generators. Usually its on for some hours each day. However, if its broken, it could be down for an extended period of time. Some hotels and restaurants and stuff have their own generator so they might have power all the time, or at least power more often.
Doesnt matter which side first. Thats up to your preference.
Boats dont overturn between Bluefields and Corn Island. There has really only been one time in many years where there was an accident between big and little corn, and that was on a day when the boats were told not to go because of the weather but the captain ignored it and went anyway
Ometepe isnt really a beach place. Its a tourist location, but its a huge island, and there are plenty of places to go without tourists or parties. SJDS and the surrounding beaches (tola, popoyo, gigante, maderas, marsella, etc) all have a reputation for being surf/party beaches. If you are looking to avoid that, you can just go to any beach north of Tola.
Not necessary to book in advance, unless you have specific tastes. Everywhere always has plenty of rooms if you arent picky. But if you need A/C or wifi or something special, then book ahead. The luxury places usually book up faster than the local place with a bed and a fan. I prefer booking as i go because it keeps the itinerary open. Never once been left without a room, even on christmas/new years.
No. Most of Nicaragua is farther than 4 hours from those cities. But as far as tourists know or care, most all of Nicaragua is within a few hours of those cities. There is a majority of the country that is 12+ hours away, and much of it on the caribbean side is accessible only by boats and animals, and can take multiple days of traveling to arrive. But for most tourists the small swath of pacific from Leon-Managua-Granada-Ometepe-SJDS is all of Nicaragua.
The rest of the country has a lot to offer, and some of the best places (in my opinion) are out there, but the difficulties in getting there, and the fact that they are not luxurious / westernized or well publicized means that they are rarely visited.