Is there a miscalculation in the Bestiary's mummy entry or is there some reason why a mummy with a +6 BAB gets one slam attack and a morhg with a +10 BAB gets two slam attacks?
What I really want to know is: would a mummy be able to use its' slam attack at +9 as part of a full attack action when armed with a longsword? Would a morhg have to forego one of its' slam attacks to also attack with a sword, or does a slam attack have nothing to do with the hands? In which case the mummy would get its' slam stack along with the sword, but I'm still left wondering why a morhg gets two slams and a mummy only gets one. Could it be because mummies are slow?
The mummy gets one because the stat block says it gets one. The morhg gets two because the stat block says it gets two. You're overthinking it a little.
Yes. A mummy can use its slam (as a secondary attack) if it uses a one-handed weapon as well. A morhg would have to give up one of its slam attacks to do the same. Slams are generally strikes with an arm unless mentioned otherwise in the entry or the creature has no arms.