... although the link title may not suffice as a trigger warning. Granny, of course, reminds me of the Cleaner in Pulp Fiction.

I can’t be sure whether you reproduce a flaw of the original story or actually lost connotations of a polish word I don’t know, but I think the guy in "But, you’re the boss… right, boss?" A man next to him asked, fidgeting hesitantly should better be introduced as "a young man"; and "a" should not be capitalized, because it continues the sentence.

... and especially how could he tremble thinking what his beautiful and petite wife was going to do ...
"How could he" is the word order for a question; since this is not a question, it should be "how he could".

Again, I’m not sure whether the ongoing tense issues in this paragraph are the original story’s fault (or Polish grammar may just be different):
Boris didn’t plan it, damnit.
Should be "hadn’t planned" (past perfect).
Since too much past perfect is hard to read, telling the backstory in past tense although it had happened before the actual story may be acceptable, but at least
She’d leave him, just like she announced... should be in past perfect again – "had announced".

I know somebody that can help, but I have a to warn you, wolfie, she doesn’t like being argued with.
If this isn’t meant to be Dora’s specific slang, "that" should be "who" (talking about a person, not an object); and remove "a".

She looked about sixty, maybe seventy years old, and was giving off the impression of a woman that he would have left his pups with without hesitation; one that he would offer to carry heavy grocery bags for…
Should be "who" again (two occurences).

Boris and Andrew grabbed it, placed on the sheet of cellophane, and Granny started her work without any excessive explanations.
Missing word "it".

Next time, if you’re gonna do this yourselves, try to separate the joins, and avoid breaking bones.
Should probably be "joints" (plural of "joint") although the plural of "join" may fit the context too. (Tomash may have said that already.)

Fifteen minutes later, Granny took care of the second body as meticulously as the first, chopping it into handy pieces, putting in a bag, and finally wrapping both bags with strong tape.
Missing word, either "it" (referring to the body) or "them" (referring to the pieces).

None of you remember that fact before the deed.
I don’t know what to make of this; it depends on what is actually meant in the original text.

• If it’s meant to be a shortcut question – "(Did) none of you remember that fact before the deed?" – Granny leaving out the "Did", the period should be a question mark.
  
• If it’s meant to be a statement, it should either be "remembered" (past tense, referring to the actual deed) or "remembers" (third person singular, because "none" refers to a single one out of an unspecified number, and present tense referring to them always slaughtering people without thinking it through.
  
• In the latter case, inserting "ever" ("None of you ever remembers ... ") might clarify that Granny doesn’t refer solely to the current situation, and probably refers to all men, not just Andrew and Boris.

Granny winked at him, unwrapping the bandana that keeping her elegant perm intact.
Again I don’t know what to do with this. Possible are:

• "kept" – simple past tense, just narrating.
  
• "was keeping" – past tense continuous, because the bandana continued to keep the perm intact at least until Granny finished to unwrap it.
  
• "had kept" – past perfect, because the bandana was removed and thus didn’t keep the perm intact anymore.
  
• "had been keeping" – past perfect continuous, discontinuing the "keeping intact".

My grandchildren will wake up in roughly to hours.
Should be "two".

She smiled devilishly, packing all of her bottles and brushes to the suitcase.
I’m not absolutely sure, but I think this should be "into".

One more day and I’d probably kill them, but then I’d have problem with the bodies.
This should either be "a problem" (singular) or "problems" (plural).

HG

Opowieść mi się podobała. Wszystko dobrze brzmi, ogólnie. Jedyny błąd który zauważyłem jest że między kośćmi są "joints" a nie "joins".

It was a lot of fun! You're a good translator! :D