Since I started critiquing the SC variants by abusing the option of a lance above I thought I ought to continue on the topic of SC variants in general and the proposed ones specifically.
First up, obviously, remove the lance option altogether. SMs should never, not ever have lances on their purpose built cap ships.
Torp variant: while I don't mind the idea of a torpedo variant, my objection in this case lies in the amount of torpedoes. Even the empowered Endeavour CL (which I would still prefer to see as 25cm/90° with 6+ prow instead) cannot fit that many torps/guns in its (substantial) front end. The Dauntless can't either. I do see the option as being on a par with the number of THs being given up, and it's not overpowered compared to the ship's cost, it's just that the SC shouldn't be able to carry that much in its prow; of either torps or THs.
Recommended fix: Drop the prow TH of the standard SC down to 1 and drop the cost down by 15 points. This allows an option to swap the TH for str 3 torps at no cost.
Dropping the number of THs on standard SCs reduces the maximum potential AC in the fleet. To make up for the loss of potential I suggest including a carrier variant (1/1/1 prow/prt/strbrd) at +20 pts (dropping broadside guns). Limit the carriers to no more than half the SCs in the fleet. This would keep the maximum potential AC the same if someone wanted to go AC heavy, but also give them the option of not going AC heavy.
I would also make the second shield a forced upgrade bringing the base cost back up to 145 pts (so a carrier is 165 pts).
So if someone took one normal SC and 1 carrier SC then it'd cost 310 pts, they'd have 4 THs, 6 BC, 2 shields each and 4WB L+R. Currently, 2 normal SCs with extra shields would cost 320 pts and give the same except +4 more WBs L+R. Losing all 4 THs they could get 12 torps, whereas with the proposed version you could only swap 2 THs for 6 torps total.
This would give 4 different variants: Guns + TH, Guns + Torps, THs + TH, THs + Torps.
As for the prow BC option, I too find it too powerful. 8 BCs are worth much much more than the 3 lances of a Dauntless. Of course, this discrepancy again comes about due to the prow heavy armament of the SC. Trading the compulsory shield upgrade for the reduced TH makes it possible to swap the remaining TH for a more reasonable alternative. Although, in this case I think that there is no swap for BC that wouldn't be too powerful. A gunnery weapon that hits like lances and is 3 times more likely to crit is just too powerful to allow them to be massed like that. Rather, a WB swap would be OK. Maybe 2WB@30cmLFR.