2. You don't need to apply for a Japanese transit visa, you only need a visa for Tahiti, but you need to prepare your valid flight information from Tokyo to Tahiti, which may be checked. If it doesn't work, you should be considered illegal. As long as you have a Tahiti visa, when you leave Beijing, you can provide valid visa information and valid information about your flight from Tokyo to Tahiti in the second half of the year to go through customs formalities. This may also be a spot check.
According to your situation, you should have bought a joint flight ticket, so you need to consult the Japanese customs or the counter in Beijing in advance about how to handle your luggage. One of my personal customers has encountered this situation, but he booked flights from Air China and Air Tahiti to Japan, and after consulting with Air China, his luggage was hung up for him.