What does happen when two wedges impact? Is it an all-or-nothing, with one wedge undamaged and the weaker destroyed? A case of complete mututal destruction, regardless of ship size, or at least within a fairly tight range of ship sizes? And if one wedge is destroyed outright, what happens to the wedge which survives? Does it take node damage, or is it completely unaffected?
When the impeller wedges of two impeller-drive vessels come into contact, wedge interference causes the nodes of the weaker vessel (or of both vessels, if the wedges are quite close together in the size and strength) to vaporize. The portion of the hull in which the nodes are mounted goes with it, and the capacitor rings associated with the nodes arc over and release all of their stored energy in the process. And if you think about the power levels routinely involved in Honorverse technology, I think you can see why this particular form of collision has a tendency to totally destroyed its victim at least as spectacularly as a breached fusion bottle.
The sole exception to the above occurs when one ship's impeller wedge is fully established, and the other ship's impeller wedge is not. There is a time period during the powering-up process for a wedge during which the wedge is actually "up" but not yet established at full power. In effect, there is an area around the ship in question in which there is a powerfully stressed gravity band which is readily observable and highly destructive to any material object in its area of effect, but which is vastly weaker than an all-up impeller wedge. You might think of it as the first stage of a multi-stage activation process. This field's existence is readily detectable, but a smaller vessel with a fully established impeller wedge could, if it entered the area of the "first-stage" field, knock out the impeller nodes producing it. Because the power levels involved are still orders of magnitude lower then those involved when the nodes go fully active and the wedge goes to full strength, the destruction is far less spectacular. Thus Honor was able to take out the nodes of the courier vessel in Basilisk without completely destroying the ship. And the fact that the courier boat's "first-stage" field had the same dimensions as its full- powered wedge would have had is why she was able to come close enough to take the field down without physically contacting the courier boat itself. Had the positions of the two vessels been reversed -- that is, had Fearless been bringing up her wedge and the skipper of the Peep courier boat had been able to maneuver his vessel, with fully established wedge, through the cruiser's "first- stage" field -- Fearless would have suffered effectively the same damage that the courier boat did.
Note that smaller and weaker impeller wedges come up faster than larger and stronger ones. This means that the courier boat's window vulnerability was actually narrower than Fearless' would have been. This also applies to missile impeller wedges, where the differences in the fundamental technology (see the answers to your second question above) also come into play. If it didn't, an impeller-drive missile would take so long to bring its wedge up from "standby" that it would be totally useless as a weapon system.