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> 俺が疑問に思った一名の欠員について、皆も何かしら不審に思う点があるらしい。

For 疑問に思った does ["〜に思うこと is equivalent to
〜に対して思うこと."](https://japanese.stackexchange.com/questions/29619/trouble-
interpreting-%e3%81%ab%e6%80%9d%e3%81%86-in-%e3%81%9d%e3%81%ae%e6%ad%a6%e5%85%b7%e3%81%ab%e6%80%9d%e3%81%86%e3%81%93%e3%81%a8%e3%81%af%e3%81%84%e3%81%a3%e3%81%b1%e3%81%84%e3%81%82%e3%82%8b%e3%81%bf%e3%81%9f%e3%81%84)
apply? Or is it 疑問と思った?

For 不審に思う, is it mandatory to read it as the adverbial of 不審, or do you
interpret it the same as 疑問に思った?
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grammar
Uses of に思う in this statement
The two structures are very similar in meaning, however:

With (〜が)〜だと思う you're stating that you think/feel (something) **is** a certain
way.

With (〜を)〜に思う you're stating **how** you think/feel (about something).

So 疑問だと思う means "I think/feel (something) is a question".

But 疑問に思う means "I feel dubiously about (something)".

Likewise:

不審だと思う means "I think/feel (something) is suspicious".

But 不審に思う means "I feel suspiciously about (something)".
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XXを疑問/不審に思う means "feel dubious about XX" "feel suspicious about XX", i.e.
"feel that XX is questionable" "feel that XX is suspicious".

> 一名の欠員を疑問に思う I feel dubious about one vacancy  
>  (≂ ～～をおかしいと思う/疑わしいと思う)  
>  その点を不審に思う They feel suspicious about that point  
>  (≂ ～～を変だと思う/怪しいと思う)

Similar examples:

> ～～を残念に思う feel sad about ~~ / feel that ~~ is unfortunate  
>  ～～を変に思う feel ~~ is strange

疑問に, 不審に, 残念に etc. here are adverbial forms, but they're not describing how
the action 思う is performed, but describing what you feel/think the object (the
thing marked with を) is.

For more, please refer to: [Confusion with に、を、と particles used with verbs
like 感じる](https://japanese.stackexchange.com/a/57223/9831)

* * *

So to answer your questions:

> For 疑問に思った does "〜に思うこと is equivalent to 〜に対して思うこと." apply? Or is it 疑問と思った?

The に in that thread has a different usage/meaning. 疑問と思った is close, but
疑問だと思った might sound a bit more natural.

> For 不審に思う, is it mandatory to read it as the adverbial of 不審, or do you
> interpret it the same as 疑問に思った?

The latter.
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