I'm not confident in some parts but try to answer.

Your interpretation seems correct in terms of meaning.

But it's difficult to answer your grammar question. I thought it is 形容動詞, but Wikipedia says there are multiple perspectives about classification of such words.

However, I think x割増しだ can be considered a kind of 静かだ, 簡単だ, 異常だ, 永遠だ, etc.. You can say "セミの声のせいで、暑苦しさが５割増しだ", just like "暑苦しさが異常だ". (You can't say "暑苦しさが何割増しだ", though. I can't explain the reason in a logical way. Someone please help!)

So, perhaps it might be impossible to complete grammatical analysis here. My opinion is you are ok because you've got the meaning of it.