I think in the early days, probably a majority of senior monks were qualified to teach bhikkhunis, so this rule affected a large group. Nowadays of course, the qualification criteria are applied quite loosely. My guess is that few ovada monks have detailed knowledge of the bhikkhuni patimokkha for example. So again, with the looser interpretation, it could potentially cover a larger group of monks. (But yes, you are right, many monks would not be affected by the rule.)
In practice though, it would be extremely rare nowadays for bhikkhunis to show up at the cave of a forest monk and expect a teaching. Ovada is done on the phone or on skype, no matter where the bhikkhunis and bhikkhus are travelling.
No, unqualified monks are specifically exempt from the duty to give ovada.