As for me, the only difference (except for the accent) between "odpowiedź" and "odpowiedzi" in pronounciation is the presence or absence of "i" at the end of the word. The same with "niedźwiedź".
And as a result, "odpowiedzi" has one syllabe more than "odpowiedź".
jedna od - PO - wiedź vs. dwie od - po - WIE - dzi
jeden NIEDŹ - wiedź vs. pięć niedź - WIE - dzi
Accent changes because we usually accent the last-but-one syllabe, and here a new syllabe at the end appears - so the accent is shifted.
most Polish nouns are accented on the first, rather than on the penultimate or ultimate, syllable in the singular.
Really? Your examples (with errors in syllabe division) show that the accent is exactly on the penultimate syllabe (or it may seem that on the ultimate one, but it's because of an error, I showed the proper version). Rarely, in words like "mateMAtyka", "FIzyka", "MUzyka" - mainly names of sciences with a Greek origin, it's the third syllabe from the end, although Polish people tend to accent them also on the penultimate syllabe. There are also other exceptions, some result from the grammar, but the main rule is the penultimate syllabe and it doesn't change with the noun declination.
Nothing is hardened here. In the plural nominative/singular accusative etc. of these words there is just "i" added at the end, and that's all. The strike creating "ź" from "z" disappears, but this is because there is "i" after it, which already indicated the softening of the cosonant, this is only a spelling issue. "Odpowiedzi" is pronounced exactly like "odpowiedźi", just "odpowiedzi" is the proper spelling and "odpowiedźi" incorrect - don't ask me why, it's just a rule that you don't put the strike when there is an "i" afterwards. Probably because "i" after a cosonant in Polish somehow forces it's softening, it's almost always so in Polish, you will not find a hard cosonant and "i" after it. The exceptions are words with foreign origin, like for example sinus (s before i is not softened, it's not pronounced as "śinus"), but not always, for example nazizm is pronounced as "naźizm" (it seems that German was too tough for us and we needed to change the pronounciation). Or there is such an old soviet brand of lorries, ЗиЛ, which we write in Polish as ZiŁ (most Europeans do it as ZiL), and here we pronounce the "z" as a hard cosonant even though there is "i" after it. But never in typically Polish words, and even not always in foreign words, where it's hard originally.