Now on the subject of the Invincible, it isn't just a weak Retribution - though the two have roughly similar firepower, they have completely different roles.
The Retribution is a Linebreaker, and you're right that with FP18 and S9 forward torps, there's nothing that's going to outgun it at close range. It excels in fleet actions, leading a phalanx of 20cm 6+ armoured prow cruisers into the heart of the enemy - The Invincible, with FP3 fewer per side and a glass chin couldn't hope to compete here.
The Invincible was meant for a different purpose - keeping pace with raiding chaos cruisers and obliterating them. It doesn't need a strong prow armament because it doesn't want to be closing with the enemy - it's too fragile. What it does have is double the firepower of an Emperor at the same range, and it can use its speed to use that range to maximum effect. In a fleet situation, its best use isn't charging into the centre of an enemy fleet, but harrying the enemy from long range either by itself, or heading up a group of cruisers that can do the same. (this is why I still like the Mercury, and the idea of a Siluria with 45cm batteries).
The two will play completely differently, and it will be perfect for a bakka list.