As you know, you should not generally rely on word order in Russian. Look:
При одном только условии устойчиво это государство: форма если добровольно принята его.
This is an absolutely correct sentence with the same meaning, just with a slightly different emphasis. Of course, some ways to say things are more common than others, some are plainly incorrect and some are more ambiguous than others, but flexibility is great.
The important fact is not the word order but that "его" has a Genitive relationship to "форма". Grammatically, the meaning would be absolutely identical if the sentence were
...если его форма принята добровольно.
Indeed, this form (pardon the pun) is "normal", or "neutral" in Russian, but the reverse order (this is called инверсия) is also common. By the way, there is another example of it in your sentence: "государство это", and you didn't stumble over it.
There is a finer issue with this sentence related to the word order: does "только" relate to "условие" or "устойчиво"? Unlike "его", its grammatical form doesn't hint us. Yet logically, the meaning is drastically different: is there "only one condition" of stability, or is stability only ensured on this condition?
Obviously, the first interpretation is false and they meant the second one. (Although, who knows...) The way you translated it seems to retain this ambiguity. Yet, I'm not sure it's right, because in English with its stricter word order you can easily avoid such ambiguity:
The state is stable on only one condition...
The state is only stable on one condition...
If the first one were correct, they would have said "при только одном условии". Here, we infer the relationship from essentially the word order, even in Russian.
But if they said "...только устойчиво при одном условии...", like in the second English option, it would mean a different thing: only stable but nothing else, say, stable but not prosperous. This is almost non-sensical, so they did what they did, hoping that we'll parse the meaning correctly.
(This is why mathematicians love the construct "if and only if").
There is one form that would unambiguously say what (I believe) they mean, but it's a bit more colloquial:
Вот только, государство это устойчиво при одном условии...
Anyway, I think it would be better to translate it "only stable", simply because you can resolve/reduce the ambiguity effectively. Also remember that when you see demonstratives like "это" in Russian, you often have the option of using "the" instead of literally translating "this" or "that".