Concise interpretation of JSON data (Location History) and altitude

Question

I got this data from my phone, most of it I generally understand, but some of the GPS positional values have me confused. The data was taken by a Samsung Galaxy S8 which has a barometer built in. I've been reading many posts and answers about this topic, but it didn't answer these specific questions.

    "locations" : [ {
    "timestampMs" : "1531422503855",
    "latitudeE7" : 399187535,
    "longitudeE7" : -751663977,
    "accuracy" : 16,
    "altitude" : -28,
    "verticalAccuracy" : 2
  }, {
    "timestampMs" : "1531422474313",
    "latitudeE7" : 399187554,
    "longitudeE7" : -751664138,
    "accuracy" : 16,
    "altitude" : -28,
    "verticalAccuracy" : 2
  },

Now I understand how to decipher most of this, the timestamp is in the epoch format, and the coordinates are a factor of 10^7. But basically, it's these the last three that have me confused. As I understand it, accuracy is defined as the radius of 68% confidence. So I'm assuming the value returned, 16 in this case, is that the GPS is 68% confident that it's accurate within 16 meters(?). But with the altitude and vertical accuracy values, I have NO idea. I know that Altitude accuracy for GPS is very inaccurate for many reasons, but I'm assuming this -28 is negative 28 meters below the sea level (or is that not correct?)? The value regardless of what it is, is also the altitude of the phone itself correct?

When those coordinates are input into an altitude check site, I get values of say 5 meters, does that mean 5 meters above sea level? If there is a building at those coordinates does that mean the 5 meters where the building is perpendicular with the street?

Second but small question, the phone recorded two GPS points within 30 seconds of each other, where the GPS basically recenters and gets a more accurate location, is it safe to assume that this second location is the "correct" location with how frequently the device polls the data?