The methods proposed for reducing the amount of interest to be paid

Here we have, "The methods {...} are not able..."

This isn't great because it's not the methods themselves that are able (or not able) to do something in this context. Instead, it's the people or things using those methods that are able (or not able) to do something.

Also, "a problem of insufficient cash flow in the future" sounds like a present tense problem. But how is insufficient cash flow in the future a problem now?

I'm tempted to eliminate (A), but I want to see if another option fixes the problems first, so let's hang onto it for now.

Same issue here: the wording illogically suggests that the methods themselves are able (or not able) to do something. This one's even a bit worse because "successfully" seems to modify "are not able" when it should modify "be used" instead.

Also, as briefly explained here, "a problem in the future of insufficient cash flow" doesn't make sense. Are we talking about a problem that exists, not in the general future, but in the specific future of insufficient cash flow? Does insufficient cash flow have its own timeline and future?

You could refer to the "future of mankind" (for example), but "future of insufficient cash flow" doesn't work, at least not in this context.

Eliminate (B).

This one avoids the "methods are not able" problem shared by (A) and (B). Also, "successfully" appropriately modifies "be used".

Lastly, "a future problem of insufficient cash flow" is clearer than what we saw in (A): "a problem of insufficient cash flow in the future." The wording in (C) makes it clear that the cash flow issue is a future problem, rather than a present one.

That's another reason to eliminate (A) and keep (C).

As in (B), "problem in the future of insufficient cash flow" is confusing.

Also, does "succeeding" tie back to "the methods", as if the methods themselves are successfully creating a problem? Or does this sentence imply that in order to use these methods, you first have to successfully create a problem in the future of insufficient cash flow?

The logical meaning is more clear in (C): "successfully" refers to the usage of the methods (by some other person/thing).

That gives us a couple votes in favor of (C) over (D), so we can eliminate this one.

(E) isn't that much different than (C), but it has the same meaning issue we saw in (D).

That leaves us with option (C), where the meaning is perfectly clear, and an option (E), where the meaning is ambiguous. So (C) is our winner.