Was the PO Box a Big Bear box, or their old town, or San Diego?

Did they use the Big Bear house seasonally, occasionally, or for a time and then realized they needed to be closer to the San Diego situation?

They lived in the condo which wasn't theirs while they owned a home, but that doesn't make the Big Bear home their primary residence. It makes it the only personally-owned residence. Was it empty when they weren't staying there, or was it in a rental pool?

What you need to determine is, if there is some/enough of the most recent 5 years of ownership where you can show it was lived in as the primary residence, even if that was broken up into segments:

"The 24 months of residence can fall anywhere within the 5-year period, and it doesn't have to be a single block of time. All that is required is a total of 24 months (730 days) of residence during the 5-year period. Unlike the ownership requirement, each spouse must meet the residence requirement individually for a married couple filing jointly to get the full exclusion."

For instance, they were in Big Bear most weekends, all summer when the kids were out of school, for Thanksgiving and Christmas and Spring breaks, on occasion they went to San Diego for a night or couple of nights at a time, to deal with the health care issues, but otherwise, went "home" to Big Bear, until deciding the commute to San Diego didn't make sense. And all of this is documented, of course: a personal calendar, some emails and texts and correspondence that reflects their schedule to check in with the parent, the care givers, the facility, etc.