Can qam by chance be used to refer to the Earth measurement of a foot (= 12 inches) as well as to the body part?
No. It is just a body part.
An 'uj is about a foot, though, so if you wanted to talk about feet of measurement you might use tera' 'uj?
On the other hand, since that’s not official, people might not understand you.
Klingons have no trouble talking about kellicams when they're speaking English, so we should have no trouble talking about feet when we're speaking Klingon.
jav foot 'ab Humanvetlh.
That human is six feet tall.
Satlho', gentlemen. I'll stick to using qam for that thing on the far end of one's 'uS.
And a kellicam is generally accepted as being about half a kilometer, correct?
Non-canon sources tend to make the kellicam approximately equal to two kilometers.