She seems to be accenting the second syllable in krzeseł. Does it sound ok?
It doesn't sound ok. In the word "krzeseł" you should stress slightly the first syllable.
Yes. I'm afraid that numerals are among the most difficult things in Polish.
Also I think so, but it is interesting trying to learn it. Thank you very much for your great Help
"pięciu" is Genitive, as needed in this sentence. "mieć" takes Accusative, it's negated here, and negated Accusative results in Genitive.
"pięć" is the basic, Nominative form. And Accusative as well.