In that case, longitude would seem to be "arbitary". The only possibility left is a relative longitude difference between sites - and even that's a very large stretch assuming that the guardians attributed zero longitude to a body according to the same rules as stellar forge (but perhaps at an offset).
Exactly. But what I find the most illogical especially with this theory is that it means that the sites had to be in very specific locations and the builders could not choose freely. For a communication network there may have been constraints in the position, e.g. depending on their technology of communication maybe requiring line of sight.
But why on earth would you let some arbitrary calculation with numbers pulled out of the blue dictate your site positions, just to hide hints to the next location?
If you want to point to other locations you put up a sign, if you do not want everyone be able to read that sign you hide it, encode it, keep it secret, whatever .. but all this take the light year distances of the stars (btw, light year is also a completely human made number, there is no good reason Guardians would use the same distance unit, except they are actually from Sol), average them, and if its full moon add something here and there .. and tada, no, a very very very large stretch.