1) An invalid commitment transaction remains a valid transaction on the bitcoin network until the funding TX output is spent.
Each commitment transaction, once signed will remain valid until the channel funding transaction output (2/2 multisig) is spent by another commitment transaction or closing transaction. Even if a newer state has been negotiated between peers, an older commitment transaction is still "valid" on the Bitcoin network.
However, only one commitment transaction can be confirmed, otherwise the funding tx output would be "double-spent".
The commitment transaction can be
invalid (channel) in the contract between channel peers (it represents and old balance), but remain
valid (consensus) on the bitcoin network, as it is correctly signed, as long as it does not double-spend.
2) The breach remedy of an older commitment transaction happens after the invalid commitment transaction has been broadcast and confirmed.
The outputs of the
invalid (channel) commitment transaction, once confirmed, can be swept by the counterparty. The
invalidation happens by revealing a revocation key half to the counterparty, with which all outputs can be swept in a breach scenario, thereby penalising the party broadcasting a false channels state. Both RSMC and HTLC outputs are revoked and swept this way.