When Lainie tell Jane she is doing a project on ancient Egyptian death mythology Jane tells her that they believed that when a person dies they are "carried to the next life on the back of a sacred hippo". This is completely made up and rather silly.
The Egyptians believed you had to earn your place in the Afterlife, it was a long and trying journey, riddled with demons and tests to stop the dead reaching their goal. Egyptians tried to win the favour of a god who would aid them on their journey.
After Slocombe drops the fake American accent, he speaks in an even faker English one.
His accent ranges up and down the country, from a broad Yorkshire accent, to cockney (London) and back, with a bit of Cornish now and then. At times sounding so unrealistic it seems dubbed.
There was no reason the Scottish actor couldn't have used his real accent, he's still British, nothing about the character would have had to change for it to be played by a Scot.