Much. Adamantine is quite simply the best metal for armor and edged weapons.
Can one guy in full adamantine take on 5 goblins at once?
If the goblins are using edged weapons, most likely. Blunt weapons mostly ignore armor, but they have a hard time killing, except for whips.
stuff
1. You are indeed limited to your embark area. Don't worry - it's far more space than you'll ever use.
2. Your dwarves need to see an area for you to see it. Get digging. Usually a 3x3 staircase going as far down as you can go will reveal all three cavern levels, and there's a good chance of a magma pipe (called a magma pool ingame) reaching up to one of the caverns.
3. You likely embarked in a calm area, resulting in the lack of wildlife. Have you had any kobolds? Either all your world's goblins are extinct, or they simply can't get to you.
4. No, you will have to add the order again. If you use the job manager (j-m) you can set jobs to be done, and the jobs will automatically be reassigned if cancelled, but you can't order a job to run infinitely from it and it doesn't wait until you have the materials to do a job to reassign it, leading to more cancellations and clogging up your message menu.
5. There are giant mushrooms in the caverns that count as trees, insofar as they produce wood when cut and piss off the elves. New trees will also grow - look around the surface and you'll see some
τ-like characters. Those are saplings and they will grow into full=fledged trees in about a year.
I would also like to note that breaching the caverns will most likely solve problem #2. Cavern wildlife is nasty.