malleability in 2-of-2 multisig payment channel
Did I make an error here somewhere in my construct?


No, you did not. This is indeed an issue for payment channels which can be worked around but reduces the security of a payment channel and increases the complexity of creating a payment channel.

Because malleability poses a problem with payment channels, transactions used for payment channels used in the lightning network make use of Segwit which makes transactions that use Segwit non-malleable. Specifically, for a LN payment channel, the funding transaction must be non-malleable; it must spend from previous segwit outputs and create segwit outputs to allow for future non-malleable transactions.