Not one of the potential investors is expected to make an offer to buy

1 and 2 are both incorrect. The subject "Not one" needs a verb, and that verb must be "is." In the example about Icelandic, that subject already has a verb ("Icelandic is . . . "). When we get to "that are compact," that phrase is simply modifying "languages." It doesn't contain the main verb. Let's look at 1 again to compare:

1. Not one of the potential investors are greedy.

The portion in italics works by itself. "Investors are greedy" is a fine sentence. However, the subject (in bold) is left with no verb. We can only use "are" to apply to investors is we have a verb for "Not one" somewhere else:

Not one of my managers realizes that the potential investors are greedy.

This clearly has a different meaning, but now that "Not one" has a verb ("realizes"), we can apply "are" to investors. (Notice that since "Not one" doesn't make sense by itself, I had to add an extra noun modifier ("of my managers.")

We could also place the main verb after the modifier:

Not one of the potential investors who are greedy will agree to the proposed terms.

Notice here that "are" works correctly with "investors," but only because we added a modifier signal: "who." By adding a modifier directly to "investors," we ensure that the verb applies to them.

2 works exactly the same way. We need a verb for "Not one," so we have to go with is.

3 is different because the word "none" is not definitively singular or plural. It takes its singular/plural status from the context of the sentence:

None of the ice cream is fresh.
None of my friends are here.

You can memorize the words that work this way with the acronym SANAM: Some, Any, None, All, More/Most.