Polish vs Russian grammatical cases (accusative and instrumental) pronounciation

I've been learning Polish for some time now, spend around 300h., while I know Russian very well.
I am puzzled by polish pronunciation of this two gram. cases. I cannot stop asking myself if these cases show signs of degradation and dissipation because they are weakly differentiated when ending on ę or ą (em is clearly defined). while russian uses clearly defined sounds in accusative where applicable by у/ю and ем/ом/ю/ой/ in instrumental.

Makes me wonder if this is really a degradation proccess that would have been finished if the language was not fixed in his status quo

I too learned some Russian, only AFTER I already knew Polish:-)

Russian cases are different enough so that merely because you've studied one of the two languages, by NO means is any sort of guarantee of either mutual intelligibility, much less "fluency" in the other!

Aspectual distinctions are nearly identical to Polish, e.g. Russian verbs of motion 'khodit', 'idit' etc.

Pronounciation however, much less basic, daily vocab?? Forget about it, if you haven't actually sat down and earnestly studied the other language! Russian, as previously observed, is far more palatalized than Polish, and it contains many sounds not found in Polish.

Once more, everyday words for "time", "look for something", etc. are NOT identical between Russian and Polish, not to mention the myriad false friends you're likely to encounter along your journey!!!

Something related to "vreme", perhaps?

Of course! By the way, the vestigial Polish form was "vremię" (or possibly "vręmie"), I'm no longer certain. My point was, that knowing Polish will not help as readily with speaking, reading, or writing in Russian as some might assume. Compare briefly below:

Russ. tjas = Pol. godzina
Pol. czas = Russ. vremya
Pol. szukać = Russ. iskat'

etc........

Another curious point is that Polish nasals have been "replaced" by non-nasal, frontal vowels in Russian, for example:

Polish: "Czytam książkę." vs. Russian: "Ja chytayoo knigu." In addition, Russian requires an adverb in such a construction whereas Polish doesn't, except for emphasis cf. "Ja czytam...", as opposed to someone else reading, etc.