I was wondering if Mako was ever going to Lightningbend against the Waterbender. Seems like a no-brainer.
The Red Lotus was clearly a larger organization that simply the 4 people that went to prison. For example, Unalaq from last season. These people were probably in the organization already.
I don't think she did Metalbend, given that the platinum shackles are unbendable even with Metalbending. I think everything she did with the shackles was either physical, or the chains being pushed by one of the other elements.
Like when she first freed herself, she didn't bend the chains, she bent the stone of cave wall that the chains were attached to.