1. For players that absolutely cannot clear the last boss, 1 orichalcum is the best they can grab, clearing 9 missions. If you're having trouble clearing the boss, but still can clear it occasionally, 2 orichaclum is very possible since you clear 10 missions to get them, but 3 orichalcum is gonna be quite a stretch already.
2. Depends. For your case, I recommend you avoid spending orichalcum on orichalcum weapons at all costs so you can save it for 4LB. Additionally, these orichalcum weapons have lower stats than standard raid weapons iirc, so there's not a huge incentive to go for them unless you really need that dual skill, but IMO it's not worth 4 orichalcum.