If using the nonrestrictive (as discussed in another answer), I would simply rephrase the sentence to avoid the possessive:
It's the birthday of my friend, Adam.
With this, there is no awkward question about where to put the possessive apostrophe. (Because it will look strange no matter where you put it.)
Your question is actually a perfect storm in which rules of puntuation dictated by grammar clash with the deeper grammar of spoken English, though rarely with the noun in your example.
Unless you are in the unfortunate situation of having only one single solitary friend in the world, in your example the proper name Adam is a restrictive appositive, that is, ...
You would use the word was because that means before, is means that they are currently your grandparents but because of their nonexistence it's not possible for them to be anything. They were once your grandparent.
You would say "John was my grandfather". This is because when he died, he essentially stopped being your grandfather, so to speak. It is like if being a grandparent of parent is an occupation, like being a carpenter. You would say
"John was a carpenter."
Likewise, you would say
"John was my grandfather."
Basically agreeing with Michael Harvey's offering: Here’s my partner Jane Doe’s, and my assignment.
But the comma of apposition is surely not necessary as "partner Jane Doe's" is a complete phrase. such as: Here is x's and my assignment, where x = partner Jane Doe. (Consider "partner" here as a title, rather than "Jane Doe" being in apposition to partner.)
Without more substantial rephrasing, it will sound strange even if it is technically correct.
It would be more natural if you simply drop the use of the possessive:
Here is the completed assignment of my partner, Jane Doe, and me.
I read through this thread and found that you really do not have an answer. The etymology of all these words, adjectives and pronouns alike, is confusing, to say the least. Additionally, it seems that there are conflicting reasons for everything. Some say that there used to be apostrophes with certain adjectives/pronouns, and some say they never existed. But ...
This seems to be a pertinent question as I haven't seen this discussed in any grammar or on this site.
When the noun phrase ends with a noun we are quite happy to put an 's on the end. This is often described in grammars as the "King of Spain's daughter" or some similar phrase and is discussed on SE here. However, since the 's can only be added to a noun, ...
You are describing an attribute of the listed item. It's position in the list is secondary to the attribute.
"The/An attribute of the first of those listed."
"The [attribute name] of the first of these."
If they are distinct things in the list then the attribute's position will be clear.
"Of the bird, the mule and the horse the color of the wings is the ...
Swan in Practical English Usage has an entry called noun + noun: advanced points. In the parts section (p360) he states:
We use the 's structure to talk about parts of people's and animals'
a man's leg -- an elephant's trunk -- a sheep's heart.
But to talk about parts of non-living things, we usually use the noun + noun
Take the analogous case of the transitive verb 'like'.
(1) I like his (x),
(2) I like him
(1) requires a NP in the argument position of 'like', a position that is modified by the possessive pronoun. (2), however, is perfectly well-formed.
The point is this. In your example, the fragment [leaving the firm] in the first case becomes a noun phrase, and ...