Possessive Form of a Proper Noun Ending in a Plural Noun Ending in "s"? [duplicate]

_{I don't think this has yet been covered in any of the other questions on similar topics. There was one other very similar question, however, it was not specifically talking about the case where the proper noun ends in a plural noun. Feel free to vote to close if I am wrong.}

What is the correct way to make a proper noun ending in a plural noun ending in an "s" possessive? This frequently happens with corporations, e.g., "Dunkin' Donuts." Should one work off of the fact that the entity is singular—suggesting Dunkin' Donuts's—or should one work off of the fact that "Donuts" is plural and ending in an "s"—suggesting Dunkin' Donuts'?

I expect that the answer might depend on dialect because some regions refer to corporations in the singular form ("Dunkin' Donuts is a company") while other regions refer to corporations in the plural form ("Dunkin' Donuts are a company"). I am specifically interested in American English, but would be interested in hearing answers for other dialects too.

All you do is listen to what people say, and then write that down. That is the only rule that matters.

The doughnuts do not make up the company.

Dunkin' Donuts is the company's name

Something that belongs to the company would be "Dunkin' Donuts' annual report"

According to Fowler, according to Truss, the 'rules' for nouns ending in s (and a subset would be plurals ending in s) are:

(1) Names ending in an '-iz' sound do not take a second s - Moses'; Bridges'

(2) Names not ending in an '-iz' sound and 'from the ancient world' do not take a second s either - Archimedes' screw; Achilles' heel

[(2b) Jesus has a poetic alternative: Jesus' disciples; Jesu's disciples]

(3)Modern names ending in s, and foreign names where the final s is not pronounced, take a second s:

Keats's poems; Davy Jones's locker.

However, this leads to inconsistencies: Athens' original games were held millenia ago; Athens's most recent Olympics were held within living memory.

Many companies, institutions and place-names contain apostrophes already as they refer to a pseudo-possessive - but others don't: Lloyd's (Insurance); King's Cross Railway Station; Lloyds (now Lloyds TSB), the bank; Kings Cross itself. One would perhaps have to write Lloyd's's employees... .

The 'rules' are broken so many times that they can hardly be deemed actually to be rules. Davy Jones' locker seems the far more common variant. And different style guides give different recommendations anyway. There is no apostrophe czar (though many pretenders).