May I please help who(m)ever is next [duplicate]

I am bringing up a rather pedantic point here, but, one that has me completely stumped. This is going to require some serious grammar knowledge.

I was in a line at a shop today and the teenager at the counter actually said "May I please help whomever is next." As a matter of conversational English, I dare say, this construction borders on pretentious. But, on deeper thought, it might be fine in formal English since the subject of the dependent clause should typically agree with the main clause.

But, then I found the following post by Edwin Ashworth on this site regarding the issue:

If one wants to be pedantic, the correct version uses whoever rather than whomever in this construction as, though the accusative is required to agree with the main clause (Can I help John? Can I help him? Can I help him who is next? NOT *Can I help he who is next? - see Fowler), the compound lexeme whoever fulfils a dual accusative (relating to the main clause)/nominative (relating to the relative clause) role. Whomever fulfils a double accusative role (Treat whomever the acid came into contact with).

I followed Mr. Ashworth's explnation until he started making distinctions between dual and double accusatives (which I didn't think existed in English). If anyone could offer a clearer explanation, I would be apprecative. Thanks.

You only have a problem if you parse the sentence as:

May I please help whomever [is next].

If you parse it correctly as:

May I please help [whoever is next],

with whoever as subject of the dependent clause, then it is clear why whoever is right.

The question between who and whom has been treated several times on EL&U. In my mind the following link provides the best answers. What’s the rule for using “who” and “whom” correctly?.

It's not such serious grammar knowledge as you fear. It can appear to be intimidating only because instances of whom, whomever, whom self, whom selves, have practically become extinct. People feel embarrassed whenever they hear or read these pronouns and feel they ought to know how to use these words in order to appear more educated. And perhaps in the case of the teenager shop assistant, who believed he was acting courteously (although it does sound affected).

*May I please help whomever is next.

If we substitute "whomever" with the subject pronouns she or he who is next i.e. anyone, and joined the expression with a "that" clause, we would have the following sentence.

May I please help she or he/anyone that is next.

Substituting she or he with the object pronouns, her or him would lead to an erroneous construction. We don't say: "Me is next"; "him is next", "her is next" "us are next"etc.. We should, however, say "I (that) am next", "you (that) are next", "she (that) is next" and so forth.

*May I please help her or him that is next. (incorrect)

Who is used to substitute the subject in the main clause, in this case; she OR he, and by tagging ever onto who we understand the meaning to be; anyone.

Thus we arrive at this grammatically correct sentence.

May I please help whoever is next.

(If I had been in the shop assistant's position I would have said) Someone else's offer of assistance might have been:

May I please help the next customer.

A grammatically correct sentence with the added bonus of not sounding pretentious in the slightest.

Tip: Never use whom, whomever, to whom etc. if you're not 100% sure of getting it right.

EDIT: Many thanks to @Bradd Szonye whose comments made me realize that the question of "whomever is next" vs "whoever is next" is indeed a much more complex grammatical one than I initially thought. I wish to qualify myself as not being a linguist but someone who strives to make the "jargon" more accessible, not only for her private students but for her own benefit too.