If you are up for challenge, here is Polish radio audition about this:
In common speech it is often not pronunced or is pronunced weakly if ę is the last letter of a word. But it depends. I hear older people use more complete ę sound, in dziękuję, for example. They might go all the way with that.
Also, if it is in the middle of a word, it gets pronunced either as the usual way or "en/em" depending on what the next letter will be. Same goes for ą. ("ow" / "on/om").
In Russian I would ask: Зачем усложнять себе жизнь? :)
Most ę-sounds are denasalised because they are easier to pronounce this way. And I have no clue why anyone would pronounce rz as ř, it's the last sound Czech children learn to utter correctly, because it's the most difficult one.
Well, according to https://en.wiktionary.org/wiki/o#Polish , "o" indeed takes locative when the meaning is "about, concerning", although it can take accusative at other times.
From https://en.wiktionary.org/wiki/z#Polish, "z" takes instrumental when it means "with".
Finally, https://en.wiktionary.org/wiki/on#Polish shows that "nim" is both locative and instrumental.