Hello! The Warriors plot is very complex huh? Well I will answer your questions one at the time.
1. I think it is doable if we just share the NPC roles. Depending on how many 'main characters' you are playing (Just one who is the leader and maybe up to three if you want them paired up), we do have the share roles of the NPC's. Of course, the last time I played this, I did play most of the NPC's (like the Thunderclan warriors, I played the main antagonist and that clan, I played Shadowclan leader, Riverclan leader, Windclan Leader, Skyclan, etc.)
So all in all, it is doable if you are VERY willing to play and/or share NPC's as well.
2. The setting will be in the lake Territories, so after the the Third book series obviously (I haven't read the warriors in awhile, so I assume the Third Series is the last xD)
3. As I said on #1, NPC's is something we just both share. I have no problem playing them (I'm a writer, NPC's is just a character that won't be too fully developed), but I do want someone who is not afraid to play them and share the roles as well. If you're worried about the personalities (and if I would get mad because it's not going a certain way), it's why we will have an OOC to talk about it a bit.
SOOO yeah xD, the RP is just for someone who is comfortable playing NPC's and can stay in communication during that.
Is this the RP you'll be ok on playing?