In practice i e. in normal everyday conversation you would not use either âwhoâ or âwhomâ for the first two. The first would read âMr. Ling is just the man I want to seeâ , the second âThe boys playing football are from Class Oneâ.
The third and fourth, which should use âwhomâ would almost certainly be spoken using âwhoâ, as most native speakers do not follow the rules regarding the use of âwhomâ.
If, as a learner of English, you correctly use âwhomâ, you would get credit for a good knowledge of grammar, but you could lose credibility as you would not sound native. In this case, you canât win !