Here's a side-by-side of the mystery man in Alyea and Lovelady, taken from the Martin film. It's the best match for their profiles I could make, both men at almost full profile.
As far as I'm concerned they are clearly two different people. The hairline of each man, particularly around the ear, are completely different, as is the shape of their heads.

The mystery man is dressed in casual clothing, certainly not that of a law enforcement officer. This means he can must surely be an unidentified TSBD employee.
He does not appear to be dressed as an office worker. He appears to be wearing a lighter coloured/white t-shirt under an off white, open-necked shirt. He is not wearing a tie.
This suggests he is one of "Truly's boys".
Under these circumstances he can only be Jack Dougherty.
Dougherty was around 6'2" tall when he was 18 years old, the mystery man is a noticeably tall person.
Dougherty testifies that by the time he reached the sixth floor the rifle had been found and the Alyea clip is taken on the sixth floor not long after the discovery of the rifle.