Clausal Difficulties

I'm really not happy with the definitions that I've seen for sentences, main clauses and subordinate clauses.

The problems:
(i) The idea that a sentence is a clause that expresses a complete idea, whereas a subordinate clause does not. This is far to vague and useless a definition for my liking. I'm sure I can come up with an example that breaks the definition. I do not want to rely on meaning to define subordination.
(ii) The definition of a subordinate clause as a clause that is an adjunct to a main clause, not a part of it. This seems to make things quite messy. I'm sure it does not lead to the clearest recursive definition. I suspect that in modern grammars subordinate clauses are
constituents of higher-level clauses.
(iii) Is a conjunction a part of a clause or not? For some reason I cannot gather, co-ordinating conjunctions are considered extrinsic, subordinators intrinsic. Now, this may make identifying some subordinate clauses easier. A sentence that begins `because ...' is readily identifiable as one. But one might also view coordinators as belonging to the independent clauses that follow them. Then a clause such as `and ...' starts to look less complete and more like a subordinate clause, which it is not.
(iv) What is a conjunction? One might say that it is just a word that joins clauses together. But then relative pronouns are often not classed as conjunctions, and they do perform a joining function. One might say that
a clause containing a relative pronoun could, were it not placed next to a main clause, stand as a syntactically correct sentence in its own right. E.g., the `who is coming to dinner' in `I know who is coming to dinner' may occur as an interrogative sentence. But then consider
`She arrived this morning when I was at work'. In such circumstances I have seen `when' described as a relative adverb. Now, one might think that relative adverbs are like relative pronouns and are not conjunctions. But `when I was at work' cannot stand alone as a sentence as could `who is coming to dinner'. We need to alter it to `when was I at work?'.
To try and make things absolutely clear: in the last case I am trying to show that a given definition applying to relative pronouns is wrong because it fails when applied to relative adverbs. This relies on the assumption that a single definition must cover both cases. That is an assumption based on the idea that, for the same reason, neither relative pronouns nor relative adverbs are conjunctions.

Any comments regarding this muddle much appreciated.

R.