During each other player's untap step, effects that would otherwise cause your green and/or blue creatures to stay tapped don't apply because they only apply during *your* untap step. For example, if you control Nettle Sentinel (a green creature that says "Nettle Sentinel doesn't untap during your untap step"), you untap it during each other player's untap step.

All your green and/or blue creatures untap during each other player's untap step. You have no choice about what untaps. Those creatures untap at the same time as the active player's permanents.

The abilities are separate and cumulative. If another creature you control is both of the listed colors, it will get a total of +2/+2.

Multiple Murkfiend Lieges are redundant when it comes to the untap effect. You can't untap your permanents more than once in a single untap step.