" the Earth's orbit passed through a cloud of comet debris "
Meteors are pieces of rock that break off comets. They continue to orbit the sun, but as the earth passes through the debris some of the meteors enter the earth's atmosphere, where friction causes them to burn up.
Meteor showers are relatively common, with the best being Perseid meteor showers which peak in August.
" you'll have seen some of Doré's pictures of sinners in hell "
Charon herding sinners onto his boat
Gustave Doré was a prolific illustrator of the nineteenth century. The illustration Wyndham alludes to may well be the 1857 picture for Dante's inferno of Charon herding sinners into his boat, ready for the one-way trip to the underworld. However, another illustration by Doré, Orlando Furioso, may better suggest the chaos and bewildering suffering that Masen witnesses.
" bearing a reputed likeness of Viscount Montgomery "
Viscount Montgomery was WW2 hero Field Marshal Montgomery, created Viscount Montgomery of Alamein