ETA:
GAH! That's a terrible post. I just re-read it, and it's too confused. I don't have the time to think this through, so I offer my apologies and let it stand. Please read with caution. Sorry.
Heh, you've picked a difficult example, to be sure.
"all that were left" doesn't sound correct,
You're right that "all that were left" isn't correct for what you mean to express:
Compare:
1. [All the milk cartons] are empty.
2. [All the milk] is gone.
So you could, then, say:
1. I had lots of milk cartons. Now I only have few, and all that are left are empty.
2. I had lots of milk. Now most of it is gone, and all that is left fits in one carton.
See? "All" can be both singular or plural, depending on what you're thinking about.
Btw, the point is
not that "milk cartons" are plural and "milk" is singular. You could say: "I had many milk cartons. All that is left of them is shreds." It's whether you think of the entity as X-n units out of X units remaining, or as a portion remaining of a whole. I.e. a reduction in the number of milk cartons ("all that were left of the milk cartons were empty") or a reduction of milk cartons to something less than milk cartons ("all that was left of the milk cartons was shreds.")
So, since you're not talking about a reduction in the number of passages, but rather about a reduction of a passage to the hint of a passage, you have: "All that was left of his passage..."
and "was the still-swaying stalks of corn" doesn't seem like it would be correct, if it's referring to plural stalks.
Well, here, you're half right. If it's referring to plural stalks, it's not correct. But
does it refer to plural stalks? That's a hairy issue in grammar, and has people disagreeing back and forth. Let's look at a simpler example:
1. The problem is the children.
2. The problem are the children.
Which of the two would you choose?
Traditional grammar says the verb agrees with the subject. By that logic, the verb's form is determined by "the problem":
1. The problem is... ---> correct
2. The problem are... ---> incorrect
It does not matter what comes after that, as the verb agrees with the subject.
Now, for an exercise, let's put the children in the subject position.
1. The children is the problem.
2. The children are the problem.
Here, the agreement-with-the-subject rule, rules out (1) and affirms (2).
So now we have, according to traditional grammar:
1. The problem is the children. YES
2. The problem are the children. NO
3. The children is the problem. NO
4. The children are the problem. YES.
See? Traditional grammar would tell you that (1) the verb does not refer to the stalks but to "all that was left of the passage", and that (2) "were" is therefore wrong.
The only problem with this is that, while there aren't many people who would say (3), there are plenty of peple who would say (2). The simple agreement-with-the-subject rule doesn't allow us to look at the problem, really. (ETA: Unless you extend the definition of "subject", which does happen, too, especially in connection with the "there is/there are" debates.)
I contend that:
(a) "All that was left of his passage were the still swaying stalks of corn..."
and
(b) "All that was left of his passage was the still swaying stalks of corn..."
are both correct, but the "were" version sounds more dynamic to me, as it is looking forward rather than backward (referring to the stalks).
Be aware that this is contested ground even among grammarians, and that there is no simple answer as of yet. For you as a writer, this means that whatever you do you will run into people who'll prefer the other way.
I say, stick with were.
