In practice i e. in normal everyday conversation you would not use either ‘who’ or ‘whom’ for the first two. The first would read ‘Mr. Ling is just the man I want to see’ , the second ‘The boys playing football are from Class One’.
The third and fourth, which should use ‘whom’ would almost certainly be spoken using ‘who’, as most native speakers do not follow the rules regarding the use of ‘whom’.
If, as a learner of English, you correctly use ‘whom’, 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 can’t win !