Some questions

Not on a expert, but increasing the damage of an AOE NP would be pretty great, so should probably NP2 him (unless you don’t use him at all)

I don't ever understand people who recommend burning 5* with damage NP and now there's this...bizarre notion among the community that burning gold servant is a legit thing.

You get huge bonus on your NP if it's NP1 to NP2, go check wiki. Usually it's about 25% or 20% increase on NP damage.

I mean NP4 to NP5, that's only a "maybe" in my book. Because there's frankly not much you can really do with RP. Unless going for that specific Mystic Code that you missed, I really don't see why anyone would ever burn their gold, lest NP2.

Non-damaging NP is another story; some of the NP tier is not very great. Tamamo gains a 500hp increase I think, on NP tier. Which is not that much. But at the same time, it gives you the benefit of charging NP over 100%.

Taiga is very offensive while Medusa lily is... simply put, "not". She's more a stall servant with a huge hp pool. Kit is fairly poor. But you weigh your options and your comp. Whichever one do you prefer more.

Sumanai is not even close to Altera.

1. According to Gamepress's Babylonia guide, the best place to farm gallstones would be the Tremors of the Sacred Mountain free quest on Mt. Ebih, but the drop rate doesn't look very good. I don't know if Salomon drops them, but I'm pretty sure that the next events will be reruns so I doubt you'll be able to buy them soon. RIP.
2. Personally I would NP2 Arjuna, having a servant at NP2 is pretty great because not only does their damage increase, but you can get overcharge (which is a life saver when the enemy has an np drain skill). But the real question would be whether there's something in the Rare Prism shop more than him so keep that in mind.
3. I haven't really used either of them, since I already have some other good lancers but I think the deciding factor would be what kind of team comp you like to play. Lily looks good for debuff/ quick teams, but I've heard her np gain and damage on quick cards aren't very good so she might need some babysitting. Taiga looks like buster/ crit, so if you have any good supports for those areas she could really do some damage, especially since she's NP5.
4. If you've already cleared Babylonia and Siegfried's been working out as your farming saber, unless you really love Attila I don't see why you should raise her. If your heart's truly set on Salter then I would just put those Saber EXP cards in your second archive and pray.

1. I dont know. Probably you shoul wait for an event.
2.Arjuna is not a good servant anyway.There are lots of better archers. Burn it.
3.Grail FSN Cu and pray for Lalter (aka Titoria)
4. Why not both? Enhance Altera and wait for Salter.