Several items have been covered here, and I have a theory on each, so here goes:
1) The Troll Hammer--The fact that Spike could not lift it at all while Buffy easily could probably be explained more easily if we consider it as an enchanted hammer that only its owner could wield (think of Thor's hammer in the Marvel comics. Only Thor can lift it). Spike couldn't lift it because he wasn't Olaf. Buffy, on the other hand, could lift it because she had vanquished Olaf and had, in doing so, inherited the hammer and the ability to lift and use it. (Its physical weight couldn't have been all that great, because it was setting on a glass display shelf in the Magic Box, which gives rise to the question of just how they got it up there in the first place if it was so heavy)
2) Slayer Strength vs Vampire Strength--Angel once made the statement that Buffy was stronger than him. Other than that, no real comparison was ever made except Luke's statement in "Welcome To The Hellmouth," when he said, "You're strong. I'M STRONGER." Obviously the Slayer isn't stronger than EVERY vampire, and it was never stated anywhere that vampire strength is relative to physical size. Spike could be as strong as, or even stronger than, Buffy. Certainly she never decisively beat him in a fight, though she did beat Angel when he was Angelus. Does that mean that Spike is stronger than Angel? Maybe yes, maybe no. Nothing was ever established on either show.
3) Spike vs Doc--Doc beat Spike for the simple reasons that Spike underestimated the much less dangerous seeming demon, plus the fact that on the relatively narrow catwalk there was no room for him to move around as was his strong point in a fight.