The videos you must have seen, are most likely official LoL streams, for events like the W.C.G or other tournaments. Those matches are ran on a special server (and by special I mean not the ordinary we usually login to play); did you notice on such matches, that the commentator had the whole map revealed? or did you notice that the commentator was actually a 6th member of one of the teams?
Phreak is usually the commentator of these matches, the following are some archived streams from some of the Tourneys, were you can see what I have explained here:
IEM WC LoL: Grand Final
National ESL League
Notice specially on the latter how on the champion selection there are 6 players instead of the routinary 5, 2 of the extra players are MOD summoners, that have a summoner spell (check while the game loads, the summoner spell the 6th player has!) that reveals the map and allows them to move freely through the map.
Now, on your second question, there is no such thing as replays implemented on the game for now, people have been requesting it along with the new map Magma Chamber; I think it should be finally implemented with the launching of Season II.