Lots of questions in your post.
Sometime ago, I posted a message asking about possible differences in MLAT results with very small changes in antenna height (1-3 feet). No conclusive replies received, if I recall correctly.
All the different sources of MLAT errors are much larger than a few feet. Try hundreds of feet when adding all the sources of error. Think of it like having an error bar of 900 feet and you increased it to about 903 feet. It doesn’t matter if the error is vertical or horizontal error. Error is error. You increased the error bar about 0.003%. Your MLAT error is then averaged with other MLAT error.
Since activating the second receiver, the number of sync’ed sites dropped substantially. The total between the two was very close to the total of the single receiver, before the activation of the second one.The frequency the receivers are in sync with others also dropped.
I then disabled the MLAT of one of the receivers. The number of sync’ed sites of the other receiver jumped back up to the previous numbers. It’s also sync’ed to other receivers more frequently.
The MLAT server has two lists. One is the number of sites that are synced with yours. The other list is of all the sites that can see a particular plane. The number of synced sites doesn’t mean a particular plane can be MLATed. The requirement is to have at least 4 sites in the specific MLAT plane list.
The number of synced sites doesn’t really mean too much. Even if you have 4 synced sites you still need the MLATable plane. As for the reason the sync site number jumped up and down was probably due to how things are being partitioned for nearby site sync.
The total number of MLATable planes should be the same since both sites. Both your sites should be on the same MLAT plane list.
And here is the interesting thing: The receiver with MLAT disabled still shows green for MLAT on the FA page, even after reboots. There is no MLAT data being sent from it, as seen in the log, only messages are being sent to the FA servers.
The FA website has a lot of caching. I think if a site stops MLAT it takes some times before the website cache is updated that MLAT was turned off. There is not “MLAT is turned off message” sent from piaware to update the website. There is a “MLAT is turned on message” though.
I think I saw a bug request to make the website more responsive on site status.