Ok, I'm going to answer your questions in order. Sorry it's taken a while, but I haven't been on the forums lately!
1. You might ask the car rental company if they can bring the car to La Pavona. There is rental car service there, and I think it's Poas Rent a Car, which if I'm not mistaken is the same company that runs out of Rio Blanco.
2. If they won't, or it's too expensive, here are your options:
a. call ahead for a taxi to meet you in La Pavona. This is an expensive option, as just the trip to Cariari runs about $40, plus another $30 to get to Guápiles, plus a good amount more to get to Río Blanco. You can pick up more people in La Pavona if you don't mind sharing the cab, and they usually pay about $6 each to Cariari. Prices are negociable if you talk to the cab driver before you get in.
b. Take the public busses as far as you can, then get a cab to Río Blanco. From La Pavona, you pay $2 to get to Cariari. There are busses direct to San José that coordinate fairly well with the arrival of this bus. There are two terminals in Cariari, the Pavona bus arrives at one and the San José bus leaves from the other. Tell the bus driver you're going to San José and sit near the front, and he'll stop at an intersection close to the San José terminal. From that stop, it's about 200 meters to your right. From the other terminal, it's about 700 meters. The bus direct to San José costs about $4 I think, and they stop at a small terminal before going over the mountain. Get off at that terminal, and a taxi to Rio Blanco is much cheaper than from Guápiles. If the bus is full, they will only stop at that terminal. If it's not full, they will also stop at the very large main terminal in Guápiles, so make sure you don't get off there.
Contrary to crfran's directions, there is no bus from the dock to Guápiles. I wish it were that simple!
3. There is a direct bus to Cariari that leaves San José at 9am, you switch busses to get to La Pavona, and then catch the boat. You get to Tortuguero around 1pm or 2pm, depending upon how much water is in the river. Leaving Tortuguero, there are boats at 5:30am, 11:00am and 2:30pm. The busses coordinate with that schedule.
4. There is always a vast amount of wildlife in Tortuguero! In January, the focus is mostly on birds, as the turtles and their hatchlings are long gone. I've been seeing toucans regularly over the past few weeks, and many species of birds are nesting because the rainy season is also the fruiting season for many plants.
I hope this info hasn't come too late to be of use! Feel free to drop by Wild Ginger if you need more info, we don't mind giving out advice even if you aren't a customer!