I find your list of the five different types of clauses to be suspect. Not only have I never heard any of those terms used to describe a clause, but you haven't listed any of the terms that are used to describe a clause—although you do name one of them outside of your list. So, it would be helpful if you could give actual examples of exactly what you mean.

@JasonBassford I'm not sure what you mean "[I] haven't listed any of the terms that are used to describe a clause--although [I] do name one of them outside of [my] list." But I've added examples. Hope that clears it up.

What you're describing as "types of finite clauses" are really just adjectival terms that can be applied to sentences, questions, statements, or other grammatical units (even clauses). Mostly, they are considered to be moods. But the actual type of clause that you gave examples of is—depending on your particular terminology here—independent, main, or finite. So, what you're really asking is if a subordinate (dependent) clause can have an optative mood.

However, I grant we may be using terminology differently. To further avoid issues with terminology, please also provide examples of "a declarative, interrogative, [or] exclamative [clause being] used as a subordinate clause."

Your analysis of the major clause types is spot-on, though 'optative' is a minor clause type used to express wishes. To answer your question: no, optatives ("God save the Queen"; "Long live the Emperor" ; "So be it", etc.) are always main clauses

@BillJ Your examples of optatives are reserved only for main clauses. But what about the 'may' optative? I've just added an example at the end of the question.

@Tᴚoɯɐuo Yes, I am. Here's an example: I wonder how generous you are.

Some examples of subordinate interrogative clauses are the bracketed elements in: "I know [where he is]"; "I know [who has the key]"; "I'm uncertain [what we can do about it]"; "It depends on [how much time we have]".

@BillJ How can can anything interrogative not have a question mark? How is: "I know [where he is]." interrogative? I must have missed that lesson [joke]. I would call these terms types of sentences or utterances. Not clauses.

@Lambie I'm no BillJ, but here're my two cents: It's not a question mark but the use of interrogative word 'where' in conjunction with the un-inverted order of he is that makes it "subordinate interrogative".

@Jason Bassford: Those are "old school" terms formerly applied to forms of the verb, which are now being used as labels for clauses employing those verb forms.

"I know where he is." may be said without there being a question at all. "Do you know where he is?" is a question.

@listeneva Well said! I'd add that main clause interrogatives typically ask questions, but subordinate interrogatives (embedded questions) express questions. Usually (but not always) they can be glossed with the formula "the answer to the question". So my first example means "I know the answer to the question 'Where is he?'" Note that subordinate interrogative clauses can function as subject, and as complement to verbs, nouns, adjectives and prepositions.

The examples given by the OP are not really clauses per se. They are utterances or sentences comprised of a single clause. So, the question is off. How is: "You are generous." a declarative utterance, used subordinately by her? It would have to be: I know you are generous.

@listeneva I would say that they are marked as subordinate by the combination of the initial interrogative word and the uninverted order for what follows. This is what distinguishes them from main clause interrogatives. Which is more or less what you said. I'm pleased to see that you have a good grasp of English grammar.

English has no morphological optative, but there are various constructions which impute an optative meaning. One uses the modal verb may, e.g. May you have a long life! Another uses the phrase if only with a verb in the past or past subjunctive, e.g. If only I were rich! Another uses the present subjunctive, e.g. God save the Queen! en.wikipedia.org/wiki/Optative_mood

Well, @listeneva. Despite some of the comments from others, I think it's perfectly clear that you understand the grammar of subordinate interrogative clauses (embedded questions) in examples like "I know [where he is]". And my examples of optatives show that they only occur as main clauses. So I think we'll treat your question as answered.

@BillJ Comments from others? You must mean me. Her classification is wrong. You can ignore me, I don't mind but you needn't be so bloody rude. It annoys you that someone doesn't agree with every word you say. You must be the grammar god.

@JasonBassford I agree Jason but was booed off the block. Those utterances are not clause types....they are types of sentences.