1. Difficult to say. Out of the next-gen home consoles, it is the 360 at the moment I think, but if japanese developers are really shifting resources towards the Wii it could also get the most (and best) RPG's. But it's also very probable that the DS will beat them all (Square-Enix is focusing on the DS more and more, Namco announced that the DS will be the main console for the Tales series...). Out of last-gen home consoles, it's the PS2 without a doubt. The GBA is a distant second, it offered a few new and a good library of old RPG's.
2. The DS; reasons are above.
3. Japanese people are not playing with PC's, therefore japanese manufacturers are not producing for PC either.
4. This question is not asked clearly enough IMO. If your talking about technical conditions, the PS3 is most likely the best. But I can also give you reasons that the Wii is the best one for RPG's, turn-based games could be controlled with the Wiimote and lower development costs should offer a wider library with more innovations because the risks for developers are not that high. A 360 supporter could surely also give you reasons why his console is the best for RPG's, so I think one can't answer this question unbiased.
