I don't know the exact damage formulas. There are so many different buffs that can interact in weird ways, and a lot that only sometimes trigger at random. I had long observed that +attack percentage seemed to be intuitively stronger than +damage percentage. I finally did a controlled test and found that in one particular situation, having Takeda Shingen of Kai in the party for the +15% attack for all allies buff increased the damage that one particular general did to one particular mob by about 27%. That's a lot more than 15%, and about in line with what I had intuitively expected. I repeated that several times to verify that it was repeatable, and intentionally didn't have any random boosts equipped for that test.

I personally use an emerald as my third port artifact, largely because I happened to get a perfect rarity one once. That's not an endorsement of emerald as being stronger than some alternatives.

I personally use a buddha sculpture as my generals' artifacts for the heal. It isn't necessarily that useful in PVP, where generals dying in a single round is common, but I think it's the best option in PVE. None of the general artifacts are all that powerful.