Should I get the LNAB changed to Unknown now, until her name is found? Is that the best way to do it?
Yes, I think so. The "Onsøen" only tells where she died. It may at best be regarded as a "Current Last Name", given that Norwegian naming practice often used "farm names" as aliases. But if this is the only evidence there is for attaching her to Peder Sjursen, I'd rather not have her entered into WikiTree in the first place. At least, there should have been built a case for it.
Also is 4 Dec her death date or burial date?
It's always the burial date, unless stated otherwise. Burial was usually about a week after death, so it makes sense to put her death in late November.