[Edi]

1) Not necessarily, no, since 5 Element Masters and Celestial Masters can each search several paths worth of sites depending on what picks they get from randoms. The lv 4 sites are so rare as to be almost nonexistent and they only appear in certain kinds of terrain (e.g. waste for fire, plains fore air, forest for nature etc) and lv 3 sites are, aside from a common one for each path (Iron Cliff, Well of Pestilence, Cave of Clouds and a few others) rare compared to lv 2 sites. So a mage with multiple lv 2 paths is far and away the most efficient searcher early on. Even lv 1 paths help find some really good sites, like the metal order towers.

2) Yes. Somewhat. The higher the frequency, the bigger the payoff since there WILL be more sites with the higher frequency. Of course, searching by mage will also be more efficient.

3) Depends, but astral gems are so useful for a lot of ther things too that it's still somewhat on the high side. Now, if you were to find The Ultimate Gateway (conj. bonus 50%), that would be a different story...

4) Too situational, depending on available paths, gem income and other needs to make any hard and fast rules.

5) Rather little. They are rare and aside from a couple of unique ones (which you are unlikely to find, all other things being equal), they aren't much good to justify the effort.