Having rafted a wagon & cow across several SGs, I believe I can offer some insight into this particular topic.

1. If the animal breaks free from its leash while on a raft, nothing will happen since it cannot actually walk anywhere. When you place it back on land, check to make sure it's still leashed by rightclicking (it'll say "Unleash" or "Leash" depending).
2. Once you have dropped 4 raft pieces together, the bigger raft can be navigated by first rightclicking and choosing "Avast!" to climb aboard. It then works the same as any boat, but movement is very slow.
3. Yes, in fact you would need to in order to transport it anywhere. The trickiest part is getting the animal (or wagon, or whatever) close enough to the water, as the pickup range for a raft is extremely small (I think only 1-2 tiles). Climb onto the raft first, then rightclick anywhere on the raft. Your cursor will turn into the 'eyedropper' icon. Now leftclick on your animal and it will be teleported onto the raft. When you want to take the animal off, repeat the the process; rightclick the raft, leftclick on the shore.
4. I'd say it depends on how large the herd is, since all you really need to think about is penning them all in. Keep one seperate. Make sure he's boxed in by building sites on all sides, except
one corner. Then use "Call Thunder" for 24 Initiative, followed by 4 Sidesteps, then "Sting Like A Bee", and finally Quell. To do these attacks, just step into that corner space and step back again before it can hit you. Time it well, and you won't take a single point of damage. Just be absolutely sure that you have
fully penned-in the rest of the herd before you start calling Thunder!

To speed up the taming process, after a successful Quell, run away a bit to lose the aggro of the rest of the herd. Once done, go back and aggro any member of the herd EXCEPT the one you're taming. Doing so will make your taming animal aggro you in such a way it does not break the quelling mechanic. Do this to draaaaaaaastically speed up this tiresome process.
Hope this helps a bit.
Last edited by Moogie on Sun Jul 11, 2010 10:49 am, edited 1 time in total.