Mutual intelligibility between Polish and Russian

Is it true that these are 'jabberwocky' languages in the sense that speakers of one will understand the function words, pronouns, case endings, verb conjugations, etc. but not nouns, verbs, adjectives, etc? I.e. Is vocabulary differences the only major difficulty of Russian speakers learning Polish and vic versa?

Yes.

No!

Maybe.

I think it's true. There are some minor grammar differences but the vocabulary is the greatest obstacle to communicate (if we forgot the history of these two nations).
Take this example: Polish 'zapominać' (to forgot) apparently means 'to memorise' in Russian!

I have to say no. Vocabulary is tricky (like in the example above), but grammar can be too. Some things can look similar or same but are not. Tricky. You really need to study those languages, it's not at all that simple.