1) This is really old news
But it's a neat trick and bears repeating.
2) All skills have to be within 15 of the recipe cap in order for you to be able to synth (or desynth!) something. The "18" you heard is the max distance you can synth something with advanced support -- well advanced support adds 3 to your skill, and you can see that 18 = 15 away + 3 from adv support.
3) Its about 40 synths per level in the veteran range. It took me about 440 synths to go 89->100 cloth. Max per level was ~62, min was ~27. Since this time I believe Wizerd replicated my experiment in the 70s/80s, reinforcing a "25%" skillup rate.
4) If you are farther than 5 from cap for a specific craft on a recipe, you won't get any skill if you break the synth.
5) If you are within 5 from cap for a specific craft, then the skillup rate seems to be the same regardless of break or make.