* Plus or minus a few orders of magnitude... haven't actually calculated this.
I've not seen anyone suggest that this was an earlier, but less-violent (somewhat less emissive) Tambora eruption. The character of the gasses sounds similar. Seems there were few reliable eyewitnesses in the area.
Maybe there was a kind of ice sheet "earthquake", where one layer of ice slid underneath of another sheet, causing a double hit of the 1815 eruption when cored?
No, the eruption happened for sure. We just don't know which volcano it was. In particular the southwest pacific has lots and lots of volcanism from comparatively low peaks in island regions that don't leave a lot of obvious geology around where we can see it.