Question regarding ～をして～言わしめる

Question

Recently I've been reading a light novel and at the beginning, there seems to be some sort of silly joke-ish "personality test" with a couple of nonsensical questions completely irrelevant to the rest of the book. One of them caught my attention and I've been trying ever since to wrap my head around its meaning, but can't seem to come to a satisfactory conclusion. The sentence in question is:

ナマハゲをして、あんたが悪だと言わしめたことがある。

The ～しめる part itself isn't anything new to me and from what I can gather, it's essentially just an old way of expressing the 使役形 found in the written language; ～をして seems to be often accompanied, serving the function of を／に to mark the person being acted upon, according to my dictionary. I also found this site giving some interesting info: https://meaning-book.com/blog/20190401171934.html

From what I understand, it's basically used with the nuance that there's an adequate enough of a reason to warrant to make somebody say something as opposed to simply state it with the ～させる form.

Back to the sentence at hand, if I were to rephrase it the way I understand it, I'd get following:

ナマハゲに、あんたが悪だと言われたことがある。

Now my first question was, is the ～をして here actually part of the ～をしめる construction or does ナマハゲをして mean "while you were out playing namahage"? Personally, I'd go with ナマハゲをやって IF I were to express that but I don't know enough on that matter to be able to say for sure.

Second question was regarding the あんた - is it referring to me, the reader or the person playing the namahage? For some reason I can justify あんた referring to either, but I can't explain why - though I do think it actually refers to me, the reader.

I'm terribly sorry if this question is a bit too convoluted - if you need some clarification I'm always happy to oblige.

Answer 1

Second question was regarding the あんた - is it referring to me, the reader or the person playing the namahage?

It's the person doing the personality test aka you. One might for example in a personality test see things like:

子供時代、あなたがよくしていた遊びは？

あなたはインドア派？　アウトドア派？

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Now my first question was, is the ～をして here actually part of the ～をしめる construction or does ナマハゲをして mean "while you were out playing namahage"?

ナマハゲをして is used in conjunction with the しめる construction in 言わしめる in the second part of the sentence to indicate what was caused to say something. Like your dictionary says, it's similar to に. It seems that you already know that しめる is basically an old fashioned causative: (さ)せる.

We can break down the sentence a bit:

あんたが悪だ - You are evil

あんたが悪だと言わしめた - You caused (someone/something) to say "you are evil"

ナマハゲをして、あんたが悪だと言わしめた - You caused the Namahage to say "you are evil"

ナマハゲをして、あんたが悪だと言わしめたことがある。- You have caused the Namahage to say "you are evil" before

Note that this is a lot different from ナマハゲに、あんたが悪だと言われたことがある。, which is the way you wanted to rephrase it. It's the difference between "I have been called evil" and "I cause people to call me evil". The latter surely seems much more evil (or at least actively wants people to think they are evil).

Also note that ~をして isn't necessary to use when you want to use ~しめる. You can just use に instead. Examples from Shonagon:

あの「石川　遼」に「王様」と言わしめた「薗田」が、大学を選んだ

上海空教官時代は教え子の予備学生に「この人となら一緒に死ねる」と言わしめたほどだった

~をして is usually used for stylistic effect (quoting directly from 明鏡国語辞典):

◇漢文訓読調の言い方。

｟語法｠下に動詞未然形＋使役の助動詞「しむ」「しめる」、まれに「（さ）せる」を伴う。

And here is an example (also from 明鏡国語辞典) where it is used with just the normal させる

「師匠をして降参させたとは大した腕前だ」

Answer 2

Now my first question was, is the ～をして here actually part of the ～をしめる construction or does ナマハゲをして mean "while you were out playing namahage"?*

In some sense yes. It is the construction of ～をして ... ～をしめる construction.

As in your link,

「言わしめる」の言葉の使い方は、「自分(相手)にそのように言わせるだけの理由・根拠・価値などがある場合」

And, the following example sentence 「ペレをしてサッカーの神童と言わしめた少年がいました」, 「料理人である私に美味いと言わしめるだけの圧倒的な料理の才能とセンスを持ち合わせていました」

Here ペレをして, 料理人である私,as an authority, are emphasized and used to certify someone has the remarkable/outstanding feature.

Now, Pere, the football legend, witnessed some boy is prodigy of football and Professional chef admitted someone has authentic talent of cuisine saying it's delicious.

So, what is 「なまはげ」? According to なまはげ/ namahage,

「なまはげ」は怠惰や不和などの悪事を諌め、災いを祓いにやってくる来訪神である。

"Namahage" are ritual visiting deities admonishing bad behaviors such as the laziness, the quarrel and to keep off the evil.

So, "Namahage" is an authority to certify one's bad behavior.

Then,

ナマハゲをして、あんたが悪だと言わしめたことがある。

means "Namahage" , an authority and deity, confirmed you were the most evil person provably ever by judging your behavior.

あんた is used when you want to emphasize you "not in a polite way".