I feel the need to emphasize that this is kind of comparing apples to oranges and that some of the axioms being taken for granted here simply do not exist. While I am not saying that this could not be the case, it is also not as likely as the information that has been presented would assume it to be.
Fundamental comparison breakdowns:
1. The damage caused by a torpedo is in part because it is an object with a frontal cross section of perhaps a square foot that could weigh up to several tons traveling at 60 knots. An explosion caused by a torpedo is made significantly worse by the fact that by the time a torpedo detonates, it's momentum has carried it
through the outer hull and into the ship. An explosion inside of a ship is exponentially worse than one outside of it. During the carrier battles of the Second World War, a near miss was not really much better than a total miss. Japanese D3A1s carrying 250kg bombs which had about half of that in high explosive did no significant damage whatsoever on American carriers on near misses, even ones that were within a few meters of the ship.
2. Breaking the back of a ship is basically impossible to do intentionally in the context of the Second World War and certainly not in the context of mundane 16-17th century technology. While magnetic detonators began to see use in the conflict, they were
absurdly unreliable for the most part. During the Narvik campaign, German submariners complained constantly about their magnetic detonators which was later discovered due to have been the
different nature of the magnetic field at higher latitudes. This was not an exclusively German problem. Those who know of the sinking of the Bismarck may also know that the British air strike that jammed Bismarck's rudder was actually the second one flown that day. The first mistook the British Heavy Cruiser HMS Sheffield for the Bismarck and landed some hits... Which thankfully in this case did not detonate as their magnetic detonators flat out failed to function. Without a magnetic detonator, you literally just have to hope that your torpedo plays nice with the current and happens to hit near the keel of the ship, which is entirely a matter of luck. Even with a magnetic detonator which could explode a torpedo right as it passes under the keel, you are still taking for granted that you know the approximate draft of the ship which is going to significantly vary based on how much fuel, ammunition, etc are aboard, and you are still taking the gamble that the current will play nice with your torpedo depth setting. If the Moniter ran into a mine, it is unreasonable to assume that it hit in such a fantastically unlikely way that would have destroyed the keel. While it is marginally more likely for a mine to hit the keel of a ship all other things equal, it should be remembered that the margin for hitting a ship and not hitting a ship is extremely small in terms of depth and that putting a mine just
tiny bit deeper in order to impact close to the keel, in the currents of the Skull River could just as much mean missing the ship entirely.
3. While many ships of that 1k ton to 2k ton displacement,
were sunk by munitions of about the size of what could fit into a barrel, a great deal also survived equally grievous hits if not worse and destroyers are thoroughly optimized against survivability in favor of attributes like speed and armament, a totally opposite design philosophy to the Dwarves. Even then, it was extremely rare for a ship to sink as quickly as was described, with sinkings that swift being reserved basically for that situation where the keel is broken and the ship snaps in two, which were only perhaps 20% of all cases. Even then there are a handful of cases of just the frontal third or quarter of a destroyer being blown off and the rest of the destroyer managing to reverse the whole way back to port. As a further example of the total ineffectiveness of near misses, the Japanese Destroyer Ariake as part of the Tokyo Express to Guadalcanal suffered
6 near misses from what were presumably 1000 ilb bombs which are again about half that in High Explosives, and despite that, she was not sunk.
NAVSHIPS A (374) SUMMARY OF WAR DAMAGE TO U. S. BATTLESHIPS, CARRIERS, CRUISERS AND DESTROYERS 17 OCTOBER, 1941 TO 7 DECEMBER, 1942 Preliminary Design Section Bureau of Ships Navy Department 15 September, 1943
www.history.navy.mil
What this link is should be somewhat self explanatory, and I'll outline the relevant parts here. I am including the losses at Pearl Harbor, but am only counting Torpedo exclusively caused losses or Torpedo and gunfire caused losses.
10 Destroyers sank under those circumstances, but 3 survived to be repaired! Among the sunken destroyers include ships that were scuttled, and also those that did not sink for many hours or even days afterwards.
The most gratuitous edge case I can think of is that of the USS Laffey, which although not impacted with an underwater form of weaponry,
survived 4 bombs of up to 500kgs and 6 kamikaze aircraft, and successfully returned to port to be repaired.
A Dwarven monitor would
surely be much more inclined towards survivability especially a passenger monitor, than any destroyer ever built by human hands.
Barring a truly unprecedented intervention by Ranald(which is admittedly theoretically possible; see the effect of Italian limpet mines on British battleships after the Raid on Alexandria), it is simply too unlikely in my mind that a conventional explosive deployed in a conventional manner could take down a Dwarven Monitor so quickly that only some 20 Dwarves were able to get to shore.