Bamspeedy
CheeseBob
3) The AI seems to not get much real value from learning them. I haven't withheld maps in a long time so I'm not sure. But when I tried it in early days it didn't seem to make any difference. This is perhaps partly because the AI can't squeeze full advantage from the info and partly because it seems to already "know" some things such as resource locations without having the maps.
In some situations you can actually hurt the AI by selling/giving them maps. Any AI units that are out exploring, work the same way as when you have a unit on 'auto-explore' (keyboard shortcut is to press the 'e' key) and they bee-line for any unknown tiles. If you give civ A the entire map of Civ B's territory and surrounding lands, then Civ A won't send any exploring units in that direction and won't meet Civ B, thus won't make contact. Of course, later on they may send a settler in that direction and 'accidently' make contact or if CivB happens to be in path between you and CivA when CivA sends some units at your territory (war or sneak attacks), but sometimes that won't be for a very long time.
In my HoF game there was 2 civs that were next door neighbors on a pangea map but didn't contact each other until like 3-400 AD, and that was just by accident when they sent a unit to attack a city I had just captured (a couple turns later I killed off the one civ, so they almost never met each other). The hard part is making sure *every* tile is exposed, because of sea tiles that are only visible from 1 specific land tile, but that isn't a problem if you have a bajillion scouts like I did and you trade maps every turn (so other AI are picking up the isolated tiles you missed).
This could work well on island maps if Civ A or B had their whole island settled, so the AI would have no reason to go there. On lower levels if you had a huge tech lead, you could get the whole oceans explored before the AI got astronomy/navigation, and then sell maps and they will have a hard time ever meeting someone, because the AI does not know how to use units for the sole purpose of meeting someone (that they know where they are!)