"Mr. Pain" is played by William Morgan Sheppard. The "Mr. Pain" in the 30-year-old 8mm film is played by Mark Sheppard, his son.
The character of Dr. Mallard is much younger than the actor David McCallum. It has been established during the series that Ducky was born in 1945 - making him 34 when the Russians invaded Afghanistan. The age of the actor is irrelevant. (Likewise, Gibbs was born in 1958, making him 7 years younger than the actor Mark Harmon.)
This is the first time that Mark and William Shepphard have played the same character. The second time was on Doctor Who
Ducky speaks of being in Afghanistan when he was young and innocent. The Soviet-Afghan War began when David McCallum was already 46.