1. How much does ranger skills "suffer" from time in civilization?

The only things that really suffer are those that are tagged with dealing with Wilderness Time. EG Camo. The others, while they don't suffer, don't get the boost of being in the wilds. Wilderness Fam will not work in Civilized spaces. There are a lot more things that you can get from this, but best way is to learn a bit on your own. Role a throw away ranger and try it.

2. How is wilderness time calculated (ticks or paces)?

I believe it's Tick based. Typically if I'm playing a ranger(Never to hero mind you, but a few to the mid 30's) if I play it right, I get fast camo by level 9-11. Best way to do this is do a few of the newbie quests, and get out into the woods no later than 4th rank if you can. Fight in the newbie ranger areas until you notice camo not having any lag, then spam it up and go to town.

3. Do bigger cities count as "more civilized" as small settlements?

You can see it in your prompt. If it says civilized(Other than the Arena which is neutral) it's bad for a ranger to hang there for too long till you have built up a bunch of Wilderness time.

4. It seems that bows are pretty rare. Is hunter a better choice than explorer? (for a newbie)

Bows are rare, but so are hunters. If you're looking for a PK ranger, this one can be a tough row to hoe. A few successful ones, but for the most part, not the choice for a bunch of good PK'ing players out there. For your first Ranger, I'd probably go Human(Dex) Forester/Survivalist, it's a really basic ranger, but will let you get a grip on some of the items that you have questions about.