I’ll have to pull out my Gloria Farley book and see what script this is. It does not look like runes.
Landing on an island when the whole place is terra incognita makes sense, easier to defend themselves on an island, easier to secure and keep it that way, less likely to be detected by casual passersby. The 'artifacts' described in the body of folklore about the so-called Money Pit haven't survived. This Pulitzer guy found a quincunx of stones, which is interesting, at least one of which had some kind of marks on it -- unknown script, pictographs, what it is is not clear.