Since there is no AI, the Emperor level might be sufficient. Further increase in difficulty would just increase corruption and research time without altering essential game features, IMHO.
Regarding traits: Saefaring is a must to have, so should be enabled to all players. IMHO, two other traits can be either selected from the existing combination or adjusted by the individual player requests for a total of three unique traits. Viking indeed are overpowered not because of traits but because of berserks and longships. Removing either of these or banning the Viking completely would be a good idea. Another solution would be to have amphibious assault ability for MDI and knights like it is in AoD Conquest scenario. It would be then a very gory and messy game!
However, if only two traits per civ are permitted, it would be not so easy to balance the eight human players. IMHO, three traits of choice can be better. These can be all the same for all players, like industrious and commerical to speed up the movement through the turns. Or may be make all civs industrious and seafaring and add a third trait of choice? Or make it even four: three default traits would be seafaring, agricultural, and industrious with another one to choose from based on personal preferences? Or give all traits to everyone?
Regarding the game speed point. Just wanted to discuss a possibility of accelearted production turned on. And if I may suggest to have high landmass. This might be a major pain in the neck for the mapmaker however. Then, increased ship movement would not be that important but still cheap harbors are essential. Don't know, this might really unbalance the game.
Last and very important question. How to handle players who are holding the save and not playing within 24 hours without warning the others? Yellow card for 10 turns; if another yellow card within these 10 turns then the player gets red card...
What then? Two red cards and the player is skipped for a turn or is replaced? Or something else? Also, posting in the game thread after mailing the save is a must IMHO. RL is sometimes rude to people but the game has to move. It cannot continue for years, the players would just lose interest.