Hello - first post back in a while!
Interesting theory, but I wouldn't say that this was the case.
Rupert would indeed be older, and of higher rank having gone from being a Sergent to a Templar.
I think the writing in the scenes between Treguard and D'Armitage in The Sorcer's Isle is fantastic.
But watching the series, and reading the books I can find no link between D'Armitage and Lord Fear.
Though I've always found it a shame that D'Armitage was never made a character in the series.
It could be like Heat almost, the two great enimies tracking eachother then finally meeting as the series draws to a close.