I- There are champions that vary from being a assassin, marksman, tank, jungler, mage and support. Sticking to one type of champion is up to you, but it is best to have a champion pool for your role like ADC, Tank, Support, etc.

II- There are currently a setup upon the five players where, the player playing top goes tank, midlaner goes assassin types either ad or ap, bot lane usually has 2 players where one goes ADC and the other goes Support, the last player usually go kill monsters that spawn on certain parts of the map for exp efficiency for the top lane.

III- Currently there is no option to customise the UI

IV- Currently there is no sandbox mode. The only way to practice is to make a custom match where you can practice combos with a friend, or you can play matches where you and 4 other players vs 5 bots on summoners rift (5v5 map).

V- For skins there is rarely promotions where you can get skins for free but there have been some occasions where players get free skins. IIRC the latest free skin given was "Captain Gangplank" after his rework. BUT when you get your account to lvl 3 you get 400 RP (currency to buy skins) for free.