I agree that in your example, as well as in Navi's, the second sentence makes it clear that the referent of "only" is "one country". Of course, the word "only" doesn't even occur in Navi's first two examples.
In Navi's (3), without the clarification provided in the second sentence, I would normally understand the first sentence to mean:
3a: There is only one country in which it is impossible to establish such an economic system.
As to (4), it appears to mean either:
4a: The establishment only of such an economic system is impossible in one country.
That is to say that it is necessary to establish more than one thing in a country, or do more than simply establish the system, or:
4b: The establishment of only such an economic system is impossible in one country.
That is, alternative economic systems must be available to be established in a given country, or perhaps in a very specific country.
Your example (5a'') is quite elegant. As with the others, though, including my own, the second sentence adds clarity.
David, you ended your last post with a quote from mine, but your failed to comment on it. Can you explain?