Everything has a counter. Space hulks are massively durable but slow. Something that could deal non trivial damage from out of its range it could wear one down. Puting together a purpose built ship should be possible.
No.
Ork BS is simply too strong and at the range required to stay out of the way our Lances are weaker.
Of course there are Nova Canons, but those kill just about everything.
Even then the ones that are big enough to require such tactics (large and up) have far too much mass for this to be effective shields or no shields.
It should also be possible to customize the on target affect. Normal ships have a single powerful shield and a durable hull, hulks have a huge number of (comparatively) weaker shields and kilometers of rock protecting their hull. A gun designed to deal more damage to a hulk would likely be more effective against it at the expense of being less effective against other ships.
Hulks are not just made of rock, they are made of the amalgamations of thousands of different ships of varying types, patterns, strengths and weaknesses crammed together in a mass that gets dense and denser.
From what we can see on the outside of Death's Harbringer it's made up primarily of nids, Imperial, ork and eldar ships, so no a gun designed to deal with rock would not be incredibly helpful.
The final issue is that hulks are simply too variable you cannot just say that's a hulk kill it.
Chaos hulks have chaos bs, orkish hulks have ork bs, nid hulks have nid bs you get the idea, there is no magic bullet for these things save hope nobody controls it, which never happens, or things that are about 100% effective against everything.
