Big fireworks depend a lot on the Cargo it's carrying, although large merchants tend to blow up a lot if you hit them just between the forward mast and the bridge, they tend to sink from one fish anyhow...
The general consensus seems to be hit them near the forward mast....except for the 2000 ton passenger/cargo don't hit those, they wont sink anyway (or takes too many fish for a measly 2000 tons)
concerning magnetic versus impact, your general rule is good enough, with the addition of the following: in heavy seas 11m/s or over, don't use magnetics, and don't try long range shots, they tend to misfire a lot more
as to the "full vulnerability" I don't use it, makes it a bit too easy for my tastes
