www.alphadictionary.com

. . . dashing off memos urging reconsideration of the project to whoever would listen.
Wrestling with Moses
Anthony Flint

Whoever, whomever . . . I thought this the easiest
issue in grammar. But this has me in a tizzy.
Perhaps it’s an issue of proximity, as when the guy
on public radio says, “Investment in securities involve
the risk of loss.”

Your bouquets and brickbats humbly solicited.

For me, it would help to know when this was written. Whom has made a comeback within my lifetime, so if it's oldish, it might have been considered proper at the time.

For current usage, if I'm ending a sentence with a preposition, I use who; Who are you giving it to? Only in formal writing or speech do I use whom; To whom are you giving it?

The book was published in 2009.
You seem to be suggesting that ‘whomever’ would actually be correct. Bouquets, I think, to you.

This is a difficult one, but I think 'whomever' would be grammatically wrong in this case (no pun intended).

"Whoever would listen" is a clause, with "whoever" as its subject, so it should be in the subjective form. The preposition "to" introduces that clause as a whole, not just "whoever".

Compare "There is uncertainty about who did it". It would be incorrect to say "...about whom did it".

But I'm OK with, "About whom did you write?" In this case the phrase subject is you and who is the object of about.

I spent months learning how to use whom in grammar school and am gravely disappointed by it loss.