Yes I agree with your 5 choices, Tashalar seems more of a solid faction than the Lapaliiya which can be represented by rebel states. I think Dambrath could be one of the factions we add later when we expand to 29 faction slots (as I've read that's possible).Out of these, I'd say Algarond, Turmish, Tashalar, Damara, and Chondath are the most important 5, although I think Dambrath would be a good idea as it would give Halruaa something to fight.
I must admit that I also have doubts about the start date. You are right saying that the avatar crisis was mostly a war fought among the dieties, but initially I thought it would be a great idea to include the avatars. Now I can see more problems than benefits with the time of troubles. Still, I want the campaign to start before the spellplague because I don't like the changes it brings. But then what about the Banehold (Bane's forces) faction? Would is still be there in 1372? Oh, and the Shadovar invades in 1372 so it's a neat date.As I was researching, I noticed a problem with the start date, as the Silver Marches wasn't formed until 1371 and Damara wasn't a faction in this time until 1359. Also if we kept the timeland we should include the Tuigan Horde, mongol-like people who invaded northeast Faerun and (...) I think 1372 would be a better start point (...)
About the Tuigan Horde, yes I knew about it but their invasion was short lasting so I'm not sure if they are important enough.
Edit:
About the Shadovar symbol, it's just my idea so I took it from nowhere, a flying serpent kind of a dragon. I''ll work on the Moradin symbol for Great Rift. I made one anvil symbol but I was not entirely happy with it so didn't posted it here.