Because that solution breaks fluff
How exactly does it break fluff? Surely the only relevant fluff is how much additional weaponry can be put on a hull - In the case of a Standard cruiser, it can have extra weaponry without increase in mass. In the case of a light cruiser that clearly has space going spare, what's preventing the extra weaponry, particularly given that it has a reactor capable of powering full broadside lance decks with power going spare when replaced by LBs?
And what's wrong with deleting the ship? The Dauntless variant you proposed is much more in keeping with what's being proposed.
I don't doubt that a Dauntless variant would probably be more effective. But it's not a Voss, which has the fundamental characteristic of 20cm speed and weak forward armament.
And we can't just delete it. No matter what we do, the Defiant is still going to exist. We could ignore it, but it would still be on an official GW document, and it would still suck. Ignoring it is an admission of failure on our part.
Assuming that you absolutely must take a line support CVL then the inescapable conclusion for this ship, since it's not allowed to have 4 AC, is that it would only be used as a direct support ship. In which case forming a squadron with a Dictator or, if given torps, another cruiser would certainly put it up there in the priority list to reload. I see nothing wrong with it having to form a squadron with another ship to get this priority.
This is an extremely good point, but torpedoes would still help in aligning it to the purpose of the ship it's squadroned with compared to Lances alone.
Give it the same prow armament as the other 2 variants, cost it at 100 pts, give it a limitation of 1/750 or thereabouts (remove the Endurance limitation altogether) and call it quits. If in squadron with a Dictator it brings it up to around CG level firepower (6AC, 8 torps, 14WB) and better survivability (more hits total and 6+ prow). It would get reloaded with the Dictator and if you need to brace then it's not so bad as forming a squadron with another ship, since it's only an extra bit of firepower that gets halved. Also, if you're near a crippled/destroyed threshold with the Dictator you can push the CVL forward to take the next attack. When joining a non-carrier obviously its main role would be to simply provide offensive or defensive fighters and adding a little to the main broadside attack and torps.
This is a combination Option1/Option3, Massive Price Break and Torpedoes. There's nothing really wrong with this, but I'd prefer to see it on a closer par to the other Voss. This actually leaves it slightly weaker overall in terms of firepower to how it is now, because 2 Lances F/L/R are worth more than 2torps forward and 2WB L/R. One dorsal lance L/R would not bring it up to firepower levels of the other Voss, but would help significantly.
Not giving it additional weaponry would mean it could only be a support ship, but giving it dorsal weaponry would give it a limited ability to operate alone with firepower of slightly more than a Firestorm if forced to.
Equivalent WB strength and this ship would lose only 3 strength each side for 20 pts less.
It's actually closer to 4WB strength each side because of proportionally higher casualties on the bombers in half strength waves. Adding dorsal weapons would make up less than half that, and would cost about the 120pt mark.