Express that an event in the past could have occurred, if not for some condition
All the conjunctions in the examples you stated are applicable in Chinese

but = 但是/但
Example: [本來][X][但][Y] = [originally][X][but][Y]

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if not for = 如果不是/如非 (literary)
Example: [如非][Y][那][X] = [If not for] [Y] [then] [X]

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unfortunately = 可惜
Example: [應該是/應該會][X][可惜][Y] = [should be][X][unfortunately][Y]

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however = 不過/只不過
Example: [原本][X][不過][Y] = [originally][X][however][Y]

X = event should had happened; Y = reason it didn't happened
Other example:
[明明][X][卻][Y] = [clearly][X][but][Y]
[本應][X][最後][Y]= [originally should][X][at the end][Y]
Y= what ended up happened instead
Depend on the context, some conjunctions are interchangeable
Examples:
[本來][X][但][Y] --&gt; [本來][X][不過][Y]
[應該是][X][可惜][Y] --&gt; [應該是][X][不過][Y]