Me too, mostly because it opens up significantly more content than you can get at solo, while keeping complete control over when and how long you play.

Plan is sound, although the math may be a tad off. I'm not sure and had a long day so not going to check it in detail. Main thing to know is you get one grantable level per two levels you gain during RAF. On the first level up, the rogue is going to take up what, 15 of 29 points. Leaving 14 for the warlock, who you should probably level above 30 before granting much since it's just REALLY easy to do with 3x XP.

In duo, most things are viable. It helps if both can do meaningful DPS, and it helps if you pick your classes to complement each other by adding damage mitigation or healing or through abilities that contribute to the other's damage output (buffs, debuffs, or complementary procs such as chill & freeze for two frost mages). Shadow/Shadow should work fine for the priests, it's like leveling a single shadow priest which is fairly easy but with more DPS and eventually with higher health/mana gain.