There is occasionally a coal bearing rock that spawns at the entrance, and there is often another inside away from creatures you can mine then sneak out.
Wilhelm is 15th, and can still get coal and silver, but it takes some patience, and you have to be willing to run away the instant it starts to look bad. Even after the encumbrance changes, he would still grab a bag and a half of coal, and 1200 pounds of iron a shot while he was getting his smith skill up. Both were too common, and much to easy to get before. Silver has been difficult to get for awhile now, even moreso now, but can still be done solo, or with moderate risk with a balanced party.
As for difficulty, If Kane and Maria had been there from the start you could probably have cleared the caves, and gotten pretty decent loot from it at that, judging by what she took from the first rubble. You didn't have a decent party buffer, and no real magic support offensively, just melee types (Corvin included).