While somewhat controversial among street artists, some of whom accused Haring of selling out.
While somewhat controversial among street artists, some of them accused Haring of selling out.
I don't see this as a choice between whom or them but rather simple grammar to make the sentence correct. In the first example, "some of whom ..." is an adjective phrase that modifies "artists". In the second, everything after the comma could be considered a separate sentence.
What makes this sentence quirky is the "while". This makes the entire first part of each sentence an adjectival phrase that modifies "Pop Shop".
With the first sentence there are phrases both before and after the comma. Two phrases do not make a complete sentence, so the first sentence is incomplete and grammatically incorrect. However, the second sentence is also wrong. Everything before the comma modifies a noun that's not even part of the sentence, so it makes little sense and is generally just poor English.
Here's how the sentences ought to be written:
This shop was somewhat controversial among street artists, some of whom accused Haring of selling out.
This shop was somewhat controversial among street artists, and some of them accused Haring of selling out.
Given the intended meaning of both sentences, there's no reason to use "while", period. Otherwise, you can use either "whom" or "them", as both convey the same meaning.
You will have to do the proofreading yourself.
you will need to do the proofreading without others to help you.
which really isn't helpful since the main point of proofreading is having another pair of eyes check.
You will have to do the proofreading by yourself.
You will have to do the proofreading on your own.
depending on context, could have the same meaning as the first sentence, or could mean you need to got to a quiet corner somewhere where you are not disturbed to proofread.
If you're doing something yourself, you're usually doing it by yourself or on your own.
Best Answer
You can't use "whom" this way because a verb is missing.
If it was phrased this way:
That would be okay. It is grammatically correct but a little wasteful using the word "returned" twice. That is why your second example using "them" is better, because the word is referring back to the returning British loyalists you have already named.