They were under suspicion. (=suspicious)
The bolded part, is it an obligatory adverbial or a pred adj, IYO. Why?
We married young. (=? when we were young)
Is married a copula? And young describes we? IMO not.
young. --> complement?
--> Is young an ellipted adverbial clause?
I jumped 10 feet.
What is the function of 10 feet in the sentence? IMO an adverb.
If 'married' is a copula, how do you account for this?
As far as I know, what follows a copula is an obligatorily present fragment of a sentence.
I jumped 10 feet.
This is how I would elicit the verb complement:
How far did he jump? That "how', does it not imply there is something adverbial to the complement?
They were under suspicion.
Where were they?
What were they? under suspicion suspicious (=under suspicion)
The function of the PP is to describe the subject --> adj.
But then, 'under suspicion' expresses a metaphorical space.
True obligatory adverbials can be used with other verbs, being semantically and logically applicable; besides, other copula verbs can take the metaphorical adverbial expressions easily, unlike verbs other than copulas:
They were under suspicion. [complementation]
They appeared under suspecion. [complementation]
They put the bin under suspicion. [obligatory adverbial used illogically]
We married. [intransitive verb; no object needed; all we know is that they married]
We married when we were young. [intransitive verb; optional adverbial clause telling us more about the subject]
We married at the age of 21. [the same as the one above]
She married a Frenchman. [transitive verb; obligatory direct object; without the a Frenchman element, the verb turns into an intransitive verb]]
We married young. [copula verb; obligatory complementation; without the young element, the verb turns into an intransitive verb]
It is the complements that determine the verb class and not the reverse, IMO. With the verb 'to marry', we can't say, can we, that the verb is a copula, so it has to be complemented. First comes the complementation which determines the verb class.
In light of this, the choice is ours to decide whether we want to complement the verb or not. We have no obligations.
Similarly, we have no obligation to attach the word to the tail end of the sentence in
We married young.
'married' is used as a copula here, because it is complemented by an adjective that describes the referent of 'we'. The copula status comes from the complementation and not the complementation comes from the copula status.
We can wipe away 'young', in which case the verb shifts verb class from M to I.
We married young = we were young when we married; young describes 'we' -- predicate adjective
We married young =/≠ We married (when we were) young; 'young' describes the time at which we got married (at the time when we were young), thus modifies the verb 'married' -- temporal adverbial optional
It seems to me that "young" in this example is what is called a "depictive secondary predicate". That is, an item that is predicated of the subject (or sometimes object, though not applicable here*) of an already complete clause, providing further information about their state (hence "depictive"). Thus I would not analyze "married" in this case as a copula - which normally mediates the main predication of a clause, and thus, as you said earlier, its complement is obligatory.
There is some debate as to whether these "depictives" are adjuncts or optional complements, which I am not qualified to go into.
* example: "she married him young" (which one was young?). Or "she ate her steak raw".