we could hear if anyone came along the trail.

New2grammar

I'm afraid 'could have heard/would have heard' would give that away.

No one is actually dropping a pin when you say "could have heard a pin drop", for example. 

CJ 

It is so quiet that if someone comes along the trail, we will (be able to) hear them.   (1)

It is so quiet that if someone came along the trail, we would/could hear them.  (2)

It was so quiet that if someone had come along the trail, we would/could have heard them.  (3)

It is so quiet that if  someone drops a pin, you will (be able to) hear it. (1)

It is so quiet that if someone dropped a pin, you would/could hear it.  (2)

It was so quiet that if someone had dropped a pin, you would/could have heard it.   (3)

None of these blocks the possibility that someone may come along the trail (or not) later or drop a pin (or not) later.

CJ 

New2grammar

The fire was raging toward us. Fortunately, there was a river separating us from it. It would have had to jumped across the river to get to us. "Would have" (Third conditional) suggests the fire didn't get to us. ... What do you think?

Note, in particular, the so ... that ... structure in the original (It was so quiet that ... ) is a complicating factor.

Secondly, the semi-modal have to in your newest example (It would have had to ...) is another complicating factor. 

Lastly, your sentence with would have isn't a very clear-cut case of a  third conditional.  There's no if-clause.  Perhaps the conditional idea can be made explicit thus, however:

If the fire had got to us, it would have had to do so by jumping across the river. 

Or did you mean to focus on this one?

If the fire had jumped across the river, it would have got to us. 

Do you see how ambiguous things can get? 

This third conditional certainly suggests the fire didn't get to us.  But it seems to me that you are adding the claim that the same sentence implies that the fire could never have gotten to us, even after the time referred to in the sentence.  This is incorrect.  The third conditional sets up a past point of view.  It states something true of some past moment in time, with no implications for what happened after that past moment.

Uttered at 6 pm focusing on the state of the world at 3 pm: If the fire had jumped across the river (before 3pm), it would have got to us (by 3 pm).

This says that the fire hadn't jumped across the river by 3 pm.  But note that nothing is said about the time after 3 pm, and it is now 6 pm.  Three hours are still unaccounted for, not to mention the infinity of future time after 6 pm.  It is not inconsistent, therefore, to add this (also at 6 pm):

But fire fighters built a temporary bridge across the river to help them fight the fire.  The bridge itself caught fire at 4pm, and the fire jumped across the river by means of this bridge, endangering all of us on our side of the river as well (by 5 pm).

 ________

To relate this to a previous example:

It was so quiet that if someone had come along the trail, we would/could have heard them.

At the time of the quiet referred to here, no one had yet come along the trail.  That's the counterfactual part.  But it's also the least important part of this particular sentence.  The implication is that, continuing through time after the description of the quiet, the same relationship continued in effect between the possibility of someone coming along the trail and our being able to hear this coming along the trail.  The statement does not imply that no one ever came along the trail after the point in time when things were so quiet.  Nor does it imply that someone did come along the trail, either.

CJ