<ol>
<li> Many of whom blacklisted her...</li>
</ol>
<p>"whom" can be used because it isn't the subject right? "Many" is the subject in this case?</p>
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<ol>
<li> The number of people which...</li>
</ol>
<h2>Can "which" be used? Technically, "number" is the subject and "people" is not. </h2>
<ol>
<li> Some african [plants, while for centuries treating common ailments locally,] are now used in western medicine now.</li>
</ol>
<p>Some african [plants used locally for centuries to treat common ailments] are now used in western medicine now.</p>
<p>Some african [,by which common ailments were treated locally for centuries,] are now used in western medicine now.</p>
<h2>Why is the 2nd one correct over 1st and 3rd? o.o</h2>
<p>[Although far more eloquent a speaker,] the incumbent made fewer concrete proposals in his campaign speech than the challenger did in hers.</p>
<p>[Despite him being a far more eloquent speaker] the incumbent made fewer concrete proposals in his campaign speech than the challenger did in hers.</p>
<p>“Many” is indeed the subject, but this phrasing could not exist at the beginning of a sentence (you could say, for example, “I didn’t like them, many of whom blacklisted me”).</p>
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<p>If “which” is to modify “people” (which it likely is), we must use “who.”</p>
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<p>The first one is wrong because “while” introduces a dependent clause if one is to use it to mean “although,” whereas we only have a phrase here. I think you have a typo in the third one.</p>
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<p>“being” is a gerund here, which requires a possessive modifier: “his being.”</p>