A Battlecruiser, at least in the time of Jutland and even into WW2 is as large as a battleship, weapons as tough but not as tough and is faster. To that end, my idea of a true battlecruiser in BFG would be a 10HP ship, with only 5+ armor, 2 or 3 shields and 25 cm speed. I lean more toward 3 shields if it is a 5+ all around ship. If 6+/5+ armor, then the shields can be lowered to 2 but I think 3 can still be justifiable.
I think the FP12@60 cm WB supported by Str 3@60 cm lances are fine since a cruiser can't hope to match those weapons and so should be scared. Those Str 3 lances is not considered barely. Applying your crit at 5+ is fine. No need to add the +1 modifier. Or you can apply the +1 modifier but keep the criticals on 6+.
And there you have your Invincible.
Battlecruisers (not just at the time of Jutland, but right up until HMS Hood, the last really big battlecruiser), also had similar crew complements.
FP12 and 3 Lances only just outguns an Armageddon, by one lance. Considering they have the same protection and the Battlecruiser takes double criticals, that's not a one-sided fight unless the Battlecruiser keeps out of range (which is a very narrow 15cm band).
So here's the argument for the 12 hit profile:
Uses same model, so is by definition the same size.
Has similar crew complement (one of the way the designers of BFG described hits)
There's no precedent for giving ships using the same model different hits (except for nurgle - but that's down to additional bloated mass, they're not really the same size.)
The 6+ prow is vital because otherwise you might as well interchange it for a Chaos battleship, they become so similar without.
Double criticals and +1 modifier adequately represent it being less tough than a regular battleship.
FP12 is not enough for a battleship equivalent - the Retribution suffered from this, which is why it was bumped to FP18@45. Instead of that, the proposal is FP15@60. As a Battlecruiser it will need the range more.