yes,they can!
These are areas more then one person or creature can attack you at the same time.
You can't attack an opponent's creature directly. you attack your opponent directly with your creatures. It's up to them to decide whether or not to block. The exception to this is if one of your creatures has the Provoke ability which means you can force another creature to block it.
One option is to swim.
yes but the damage from the first attack doesnt add on to the second attack
Yes, in Magic: The Gathering, you can copy a legendary creature, but if you have two or more of the same legendary creature on the battlefield, you must choose one to keep and put the rest into the graveyard.
Yes, in Magic: The Gathering, you can block more than one creature with a single creature if it has the ability to do so.
That only applies to some monsters for example if your fighting a "moss giant" in the varrock dungeon you and your friend can attack that same monster. But if you're in the karamja dungeon where the most common place to kill lesser demons only one person can attack a lesser demon at a time. Most players don't like it when you attack the monster they are already attacking so try to stick on your own. But if the player is being a jerk i attack their monster for fun.
Yes, it is permissible to include more than one legendary creature in a deck, but you can only have one of each legendary creature on the battlefield at a time.
Only if it has an effect that lets it attack more than once per turn, like Asura Priest does.
In Magic: The Gathering, you can copy a legendary creature, but if you have two or more of the same legendary creature on the battlefield, you must choose one to keep and put the rest into the graveyard. This is a rule to prevent players from having multiple copies of the same legendary creature, as they are considered unique.
No - one of them is a mystical creature that never existed, the other is a big dinosaur like creature from the movies
You have to keep two things in mind. 1 - Normally, you only get one Combat Phase, 2 - All monsters attack at the same time. Untapping a monster after it has attacked will in no way allow it to attack once more, because you are past your Combat Phase. Even if you were still in the Combat Phase, all attacks are declared simultaneously, and you are past the 'declare attackers' step.