I actually always assumed it was an Ugnaught due to the voice tone. Stomtroopers were human and the voice sounded more like an alien voice.
I do like the idea about Dengar blasting him.
As far as the explanation of "Stomtroopers...". I always thought that maybe C3po could sense what was happenning around him and saw them after he was blasted. Or that there were some Stormtroopers with Dengar when he was blasted.
I guess we'll ever really know.