I've recorded my estimates for the ritual numbers, you can reveal away.
Every lantern you want to maintain like this needs someone to refill its oil several times a day. Might be easiest (and cheapest) to do this as close to the time you're going to use it as possible.Fire lit from a volcano. (At this step, some redundancy is easy. Light 2-3 lanterns while you're there and keep them in separate locations.)
Also, I understand that Elsa needs to stay behind to do her research, but that doesn't mean she can't get involved in the planning of where and how to acquire these things. The other PCs have a lot on their plate; if you're going to ask favours of them it helps to make those favours easy.