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> 社長に議長に田中氏が任命された。

i think the possible meanings are:

>   1. Mr. Tanaka was appointed by the chairman to be the president.
>   2. Mr. Tanaka was appointed by the president to be the chairman.
>

Is that all correct?

Using the idea in this thread [The grammar of “虎から食べられた” and
“虎に食べられた”?](https://japanese.stackexchange.com/questions/26199/the-grammar-
of-%E8%99%8E%E3%81%8B%E3%82%89%E9%A3%9F%E3%81%B9%E3%82%89%E3%82%8C%E3%81%9F-and-%E8%99%8E%E3%81%AB%E9%A3%9F%E3%81%B9%E3%82%89%E3%82%8C%E3%81%9F)
is maybe this a possible remedy:

> 社長 **から** 議長に田中氏を任命された。

making it unambiguous and meaning

> Mr. Tanaka was appointed to be the chairman by the president.

Does changing " **に** " to " **から** " both sound natural and remove ambiguity?
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passive-voice ambiguity
"社長に議長に田中氏を任命された” is ambiguous?
As you said, this sentence is ambiguous as Japanese grammar.

As my point of view, I modified it like below.

社長 --> subject  
田中氏 --> a target person  
議長 --> An assigned post

The president appointed 田中氏 to the chairman.「社長は議長に田中氏を任命した」

At first, I think this is not the passive sentence. And usage of a preposition
is importance. 「は」「に」「から」「を」... and so on. Because it is possible that the
meaning change dramatically.

Please refer a site below. <http://nihongoichiban.com/home/japanese-grammar-
particles/>

Hope it helps.
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Yes, you're right. 社長から議長に田中氏を任命された is a solution to get rid of ambiguity.
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