It is commonly asserted that the subject of a sentence is the noun or pronoun that does something or exists in a particular state of being.
First, this is not true. Oh, it's commonly asserted, certainly; but that's not what Subject means.
Therefore, in the sentence All but Jones are here ...
Sorry, there's simply no therefore about it.
You're working from a false premise, so you can prove any proposition you please.
Subject is a grammatical term, with a grammatical definition.
That means its definition cannot refer to what it means -- that's not grammar.
It has to refer to how it's used grammatically, which -- in English -- means syntactically.
There are a number of syntactic tests for
One of them is governing verb agreement, as you suggest.
Another is inverting with an auxiliary verb in questions:
- The prisoner is still being held.
Is the prisoner still being held?
- What he told you yesterday is still being denied.
Is what he told you yesterday still being denied?
Still another is B-Raising with Passive; this identifies the subject of an infinitive clause
- They believe [the prisoner to be guilty].
The prisoner is believed to be guilty.
- They believe [there to be gasoline stored here].
There is believed to be gasoline stored here
As can be seen, subjects can be much more than one word; as for meaning,
quite often the subject has no meaning, like Dummy There in the examples above, or
Dummy It in It's raining or It's a long way to Tipperary.