It means "attribute to" / "blame on". It's used rarely, mostly in academic contexts and the vast majority of native speakers do not know what it means. That said, you used it well and so well done. Most native speakers would say "blamed on" or informally "put down to" here.

It's correct and it means "caused by", although it's not a word that I see used very often. If you type impute on Google you will get different meanings and examples of usage.

Doesn't sound right. Probably better used only in written language.