Learn English – ‘Who is who’ or ‘Which is which’

A.1. To rephrase well using "who," you need to know a little more about the relationship between the President and the team. Possibilities might include: "Speaking as the President, who chose the team and leads the team" or "Speaking as the President, who chose the team and acts on the recommendations of the team," etc. More generally, if you want to use "who" and not "whom" you need to know what the President does with or gets from the team and not just what the team does for the President.

B.1. To me this sentence is ambiguous. It could mean that for most people e-mail has become a necessity, or it could mean that there is a subset of people for whom e-mail has become a necessity, and for most of that subset the solution is not available. For the former I would add a comma: "That is not a solution available to most people, for whom e-mail has become a necessity." For the latter I would say "That is not a solution available to most of those people for whom e-mail has become a necessity."

C.1. Yes, it means they are told how to vote. The phrase after the comma is reinforcing the point by saying that the people (the same people who are told how to vote) are forced to vote by people who have a hold over them. Arguably these are two slightly different points: they are forced to vote (versus staying home and not voting) and they are told how to vote. As you can tell from what I've just written, I think you can re-phrase to say "how to vote" instead of "for whom to vote." It is idiomatic in that it does not mean "told how to mark the box or pull the machine lever," but "told how to vote" is commonly used to mean "told which way/for whom to vote."

The best I can tell you is that in is correct while of seams to be wrong.

Cambridge explains the use of "increase of" as a noun here: Nouns and prepositions from English Grammar Today

• There has been an increase of 200,000 in the city’s population in the last ten years.
  (Of is used before numbers and quantities after increase/decrease/growth; in is used before the thing affected by the change.)
  Not: … an increase of the city’s population …

"an increase in" indicates growth of something that gets bigger in number or volume. (Longman Dictionary)

Based on what is known I can assume that a possible correct structure of the 3rd sentence using of can be the following:

• A is increased in people with PQR. This *increase in people of A * is particularly significant.

You could restructure it and drop the of (in) A part to stay clear of unnecessary problems:

• A is increased in people with PQR. This increase is particularly significant.