Sure, you can use unless in hypotheticals. Whether or not the situation has materialized has no bearing on whether or not you can use unless. Indeed, the purpose of unless is creating conditional clauses. Consider:
- A unless B
- Unless B, A
The constructions are ordered differently but have the same meaning; if one structure is transformed into the other, the semantics are preserved. In both cases, the statement says that B is a necessary condition for A; A is false if B is false, and B is true if A is true. However, saying that A is true if B is true a logical error. For example, your examples:
A (getting to the meeting on time) is false if B is false, because this sentence tells us the only way to get to the meeting on time was to catch an earlier train (which is if A then B). But catching an earlier train does not guarantee arriving at the meeting on time; what if I forgot my briefcase and had to go back?
Here, A is I should expect his help and B is he's my friend. This sentence says don't expect help from anyone who's not your friend (if not B, then not A). Note that just because he is your friend does not mean you should expect his help.
Notice that in sentence 2, B doesn't include not. Using not or not depends on the content of the sentence. Observe:
We will go to the park unless it rains.
Unless it rains, we will go to the park.
Here, B is logically not raining, because the statement means if we don't go to the park, it must be raining.
If A then B and if not B then not A do in fact have the same meaning. For A to be true, B must also be true allows us to deduce that if B is false, A must also be false. See Wikipedia for further explanation of the logic of implication.
But the discussion of predicate logic is beyond what you've asked. Simply put, it's perfectly OK to use unless for imaginary conditionals. As for the sample sentences, your use of unless is correct. They don't sound quite natural, but in both cases it's a matter of conjugation, not conditional clause structure. Here's how I'd change them:
- I couldn't have gotten to the meeting on time — unless I had caught an earlier train.
- Unless he is my friend, I shouldn't expect his help.