I know most people here have answered your questions but I just want to say a few things about the nav, first of all I absolutely love the thing, being able to find egg moves so early within the game is a dream for someone who loves playing through the story over and over. Secondly, I caught a shiny zigzagoon my search level was around 350 and my chain was around 170. A couple of comments about that, make sure when chaining that you take caution with NPC's if they move onto a space where your pokemon is hiding then it will break your chain. Also when you're chaining I recommend using a search point, this helps regulate where the pokemon will pop up (minimally 6 to 7 spots away). Farming leppa berries helps with pp of strong/helpfu moves. I use sweet scent to beef of the search number before chaining, also I was able to use sweet scent mid-chain without breaking the chain, although I personally believe that the two are unrelated, it may be possible to build up a chain and then use sweet scent to trigger a horde with higher shiny chances. Anywho I've gathered as much info on the dexnav as soon as I could, if you find anything shockingly concrete on shiny rates or other data numbers please let me know! :)
p.s. When chaining if your search result comes up with nothing don't fret you can run around until the dexnav picks up something on its own or until you can search and find what you want, so long as you encounter and ko/catch it as usual your chain will continue.