I think heed is still quite common, at least as a verb:
He did not heed the warning signs.
Even as a noun, it’s not that rare:
He took heed of the warnings.
Also, programmers sometimes use “underflow” as the opposite of “overflow”, especially in relation to stacks. An underflow occurs when you attempt to take something out of something that is empty.
I regularly use gruntled, usually after a meal. Feck is often used as an alternative to fuck, usually to indicate that the utterer is Irish. And isn’t an antithesis of inept apt?
The observant reader will have noted that tithesis is an incorrect formulation: antithesis is anti + thesis. So the antithesis of antithesis is thesis.
I’m from the North-East of England. Canny is often used. It is a multi-purpose wonder of a word and it does (or can) get used as the opposite of “uncanny”
According to the online dictionary Canny means cagey or shrewd, Uncanny means eerie or mysterious. So (just to nitpick) I don’t think they are opposites.
Inert? Never heard of anything described as “ert”.
With regard to “ruthless”, there’s a Milton poem that includes the line,’ “Look homeward Angel and melt with ruth”, so “ruth” as a word meaning compassion was in use at one time.
In the north-eastern dialect, “Canny” can also mean “good” or “fine”, normal or even superb, depending on context. That’s the usage I’d see as being opposite.
And as well as your above definition, it can also mean miserly.
Heedful is pretty common (Chrome’s dictionary recognises it for a start!), usually as heedful of X; I cannot say the same for ruthful - I don’t recall ever seeing it in print.