Added.

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Soap is rather treated as a mass noun.

That's... difficult. "Nie używałem" would be my first thought - I never used to use pink soap, I just never used something like that.

"Nie użyłem"... sounds as if there was some specific bar of soap and you're telling someone: "I assure you, this bar of soap hasn't been touched, I have never used it!" - so basically, someone assumes that you did. That's at least my impression.

Yes, of course.

Depends :)

https://en.wikipedia.org/wiki/Polish_notation