(Is this reasonable?)

It's certainly what I immediately assumed.

Light Riddle

Not actually Riddle, but yeah.

He found that the Voldemort-path was much more preferable on account of the loyalty he could obtain via the Dark Mark. The Dark Mark was so effective that the loyalty he earned as Light Riddle seemed negligible by comparison; thus he complains that he got no help from his 'allies'.

Amelia claims that Quirrell's Yule speech calling for a Mark of Britain / Light Mark "struck her as familiar", and was one of the clues that brought to mind the vanished Noble Hero.

If the above is right, then Voldemort's disfiguration would have to be a disguise rather than real damage from Dark Rituals.

Or he could have been possessing the actual body of his former classmate.