Is there such a word as 'terrorisms'?

You can have different forms of terrorism, so theoretically it is countable, but since terrorism is used as a blanket term to describe various incidents of terrorist activity, it stands as a plural without the need for the s.

Incorrect that words ending -ism cannot take plural form. Neologisms for example. It depends entirely on the sense.

Not really. You're trying to make a plural out of a singular word and it does not work that way.

Yes. The definition is "The United States of America". Another definition is "The State of Israel".

No! Any word that ends in ism cannot be pluralised.