No, not all terrain has Cantrip. That is a special thing for Shallow Sea. There are several different terrains in this set, and currently it's possible for any Mage to grab a terrain without having to pay triple cost for it. No their is not a terrain for every school, but future sets are going to keep adding to terrain so it can be even more well rounded.
As for Vine Markers, those are standard conjurations and so currently the are not replaced by terrain. In the future other Mages will certainly have their own "unique" terrains. In this set, Shallow Sea is the only terrain limited to a single Mage.
At the moment you only need to include terrain in your spellbook if there is a specific terrain that really enhances your strategy or if there is a specific terrain you're afraid of. As more terrain comes out that will all still be true, but the options will be greatly increased. We're excited with what we can do for terrain. Terrain is meant to be a more permanent metamorphosis than some of the standard conjurations. Hence, having a very specific way to be destroyed. On Wednesday, I'll be putting up a poll to see what the next card you'd like to see is.
Sounds awesome!
As for at least 1 terrain that no one (currently) pays triple for, all that really means is there is at least 1 Water terrain (that isn't Siren only), or perhaps at least 1 Nature or Mind or Earth or Air terrain.
...
"Air terrain" sounds like an oxymoron, but I guess it could be a foggy swamp or something. *edit* Oooh... Poison Gas terrain would be Air...
I will be thoroughly disappointed if there isn't a War terrain called "High ground."
This allows a duel between Siren and Paladin where they basically argue:
"This is high ground."
"No, it's a shallow sea."
"You're mistaken. This is high ground."
"You are incorrect. It is a shallow sea."
"Hmmm. I've run out of terrain cards. You're right, it's a shallow sea. My mistake."