Lamb of God is the titleThat there is an exception to your rule is proof that being sinless does not equate someone to being God. Not useful to support God coming to earth as a man in that case.
As far as Jesus being the Lamb, we have more than one example of the Lamb not being God, but rather a man distinct from God Almighty. See John 1:29,30 where the Lamb is defined as a man. See Revelation 21:22 where the Lamb is distinct from Lord God Almighty.
God doesn't need to be justified, but Jesus did. Think about it.
So there is still no proof that God came to earth as a man.
Earlier in Revelation Jesus is speaking to the churches, which is also the Holy Spirit speaking to the churches. Jesus is also given terms used for God almighty..'alpha and omega '. Jesus is on the same level as the Father and Holy Spirit
So Lamb of God would be akin to God coming to earth to be the ultimate sacrificial Lamb. Since the rest of Revelation shows Jesus' full diety.

Jesus’s Justification and Ours
Justification is God’s last-day verdict about us that’s been brought forward for us to enjoy this present day.