Issues of homestead property tax are local. Typically county and some will be state, provisions. Not IRS. The IRS only cares about primary residence, and any land that was being treated not as part of the residence, such as timber sales or tree farm, because there is going to be income tax reporting history that separates that land from the residence.

I don't see where you ever answered the issue of if this is a couple, a widower, someone was a co-owner who died (common law or community property state) and I don't see any comments about if you checked with the County records.

You might be surprised at what is stored at the county. In general, there might be a building permit that included the projected value of the project. I was recently researching a FL property where my client was the second owner, but owned since 1967. All the records were available online, because FL is a disclosure State, unlike Montana.