League-of-legends – Is the lane bots don’t play predetermined

Roaming has become a popular addition to the metagame strategy, wherein one player does not lane nor jungle, but rather simply moves through the jungle/river to initiate ganks on lanes from level 1 and onward, until the teamfight portion of the game begins.

A roamer essentially guarantees that you're either running a solo lane, or two solo lanes if you have a roamer and a jungler. I've also seen a double roamer game played, though this tends to be less effective as enemies who are wise start playing defensively and reduce the effectiveness of your roamers, which put them behind in both CS and levels.

The qualities of a roamer can vary, but the general requirements are that they are mobile, can do burst damage, and have a strong stun or position altering (push, throw, etc.) ability. Taric, Evelynn and Alistar are all very popular choices for roaming. I've also seen Sion and Blitz play roaming effectively, but they were both out of a lack of a jungle spot on the team and normally suit better in a lane.

This strategy is similar to the "double jungle" mindset in that you get three solo lanes, with the exception that there is extra pressure on getting early kills to succeed. A definite risk vs. reward tossup, as successful roamers will quickly catapult your laners ahead in level / CS as well as demoralize your opponents, but unsuccessful roamers will fall just as far behind due to the lack of their presence in the laning phase.

What happens if a full team leaves the game

The game keeps going, 0v5.

The opposing team will just have to destroy your nexus to end the game.

You can see this in the following pro match, where one of the teams decided to surrend before the 20 minutes mark, having had a really bad early game :

What happens if all players in the game leave

This one may be a bit trickier to answer, as there is no mean to trace the game once all player left.

Based on the behavior if a full team leaves, I guess the game would still continue, until one of the team has his minions destroying the ennemy nexus (which will happen at one point, try it in a custom game).

As suggested by Lyrion, it seems that if all 10 players are disconnected from a game (or leave the game on purpose), they will not be able to reconnect. This could imply that the game has been ended. However, this observation was made in a custom game, and this behavior may be specific for these games.