How to write the possessive of a proper noun that ends in a plural noun?

Question

Say we have a hotel named the Springfield Arms. The name itself is singular, since it refers to an individual hotel, but it ends with the pluralized noun “Arms”.

What is the correct way to write the possessive of this hotel’s name? Do you treat it as a singular noun overall ("the Springfield Arms’s address"), or as a plural noun ("the Springfield Arms’ address")?

I know the issue can be sidestepped by rephrasing, such as with “the address of the Springfield Arms hotel”, but I’m still curious what the actual answer is.

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I found a couple similar questions (Possessive for name with implied plural, Possessive Form of a Proper Noun Ending in a Plural Noun Ending in "s"?), but the former was closed without answers and points to the latter, whereas the latter’s answers vary widely and I can’t discern any clear rule or convention. The latter is also closed pointing the a third question (see below), which doesn’t address mine.

To clarify, this question is not a duplicate of Which singular names ending in “s” form possessives with only a bare apostrophe? — the two questions are about entirely different things. That question asks which singular names like “James” and “Aeneas” are subject to exceptions to rules about possessive forms with regard to sibilants; my question asks whether a compound singular name ending in a plural noun should be treated as singular or plural overall. That is not the same at all (for instance, regardless of the possessive rule used, there is no question that “James” is a strictly singular name).

Answer 1

This is between you and your style guide of choice.

Mine is The Chicago Manual of Style, so I would go with the rule for “a plural form ending in s . . . even though the entity is singular”: the Springfield Arms’ address.

7.20: Possessive of nouns plural in form, singular in meaning

When the singular form of a noun ending in s is the same as the plural (i.e., the plural is uninflected), the possessives of both are formed by the addition of an apostrophe only. If ambiguity threatens, use of to avoid the possessive.
_{politics’ true meaning
economics’ forerunners
this species’ first record (or, better, the first record of this species)}

The same rule applies when the name of a place or an organization or a publication (or the last element in the name) is a plural form ending in s, such as the United States, even though the entity is singular.
_{the United States’ role in international law
Highland Hills’ late mayor
Callaway Gardens’ former curator
the National Academy of Sciences’ new policy}

_{Source: The Chicago Manual of Style (login required)}