OK, nie powiedziałem o wszystkim.
Jeżeli w zdaniu z "niby" nie ma tej części z 'ale' (seemingly...but), to 'niby' może znaczyć "actually, apparently" (=it seems this is what actually happened, what the situation is really like)
1. Niby powiedziała ci to?
She actually said that to you? =Please confirm that what you've said is true
2. Ona też niby powiedziała mu, że ktoś do niej dzwonił i ...
She also apparently said to him, that someone 'phoned her and... (?)
- yes, niby = 'as far as I know' or "so she says", "so they say"
3. niby literatura, niby witryna...
it depends. If there's an 'ale' or 'a' following, the 'niby' means 'seeminlgy'. If it is the complete answer to the question "Co to jest", it may mean "looks like literature, looks like a web site"
4. Interesujące, a niby nie możliwe ! Interesting, but seemingly impossible!
Yes. Or "though seemingly/apparently...". or "and they said it was not possible"
5. Niby fachowiec z niego, a kranu nie umie naprawić
He says he knows his trade (=seemingly he's a pro), but he can't fix a tap (faucet in American English, right:-).
6. Niby jak on nie przyjdzie, to ja mam to zrobić?! Niby czemu?
So you say that if he does not come I am to do this! Explain why?
Here 'niby' means something close to "I don't believe what you say" - similar to No. 1, innit?