Possessive function of a business name which is made with a possessive

Garner's fourth edition, page 714, states regarding the name McDonald’s

It is quite defensible to write McDonald’s dinner combos (the name functioning as a kind of possessive)

On what grounds can a business name which is made with a possessive function as a (kind of) possessive?

Secondly, is the same true for a business name created with a plural noun, therefore ending in -s'? (I am not a native speaker, and unfortunately cannot come up with an example right now)

If the possessive is a part of the business name, it doesn't make sense to reduplicate the possessive:

(x) McDonald's's dinner combos

(x) McDonald's' dinner combos

McDonald's dinner combos

Furthermore, eliminating the possessive would misconstrue the name of the business:

(x) McDonald dinner combos

(x) McDonalds dinner combos

In other words, since the business name already takes the form of a possessive, the default name and the possessive have the same form. Whether it's read as a possessive or as an attributive noun (a noun that modifies another noun), "McDonald's dinner combos" makes sense.

Incidentally, Garner uses these terms (including referring to the attributive usage) in a blog post on the subject:

... the best solution is to rephrase {the french fries at McDonald’s [not McDonald’s’s french fries]} {the closing hours for T.G.I. Friday’s [not T.G.I. Friday’s’s closing hours]}. But it’s also defensible to write the McDonald’s french fries (with the name functioning attributively)—and perhaps even McDonald’s french fries (though this is strictly illogical).

I don't know of any businesses that employ a plural possessive in their name. If they did, however, the answer would be the same: use the form of the name whether it is treated as a noun, an attributive, or a possessive.