is it also for numbers to be declined the same way as other nouns?
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
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.
What is the diference between pięć (five) and pięciu (five) ?
"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.