Is it possible? Yes
Does it work? Yes
Does it work good? Eh..depends.
It really comes down to the quality of wiring and electrical noise. If you are talking about a home, then it can work decently. If you are talking about an apartment/Business then it works a whole lot less good and you are better off going wireless.
but in my case, there is already a "powerline connection" between the router and other clients... if possible, how does the powerline adapter differentiate packets from the modem and within the LAN?
The one-to-two should work, and any combo of linkage you mentioned will work. I only have a pair of D-Link DHP-303 200mbps, the actual fastest speed ever was 10MB/s, 4ms latency. Depend on your link, I had to make sure there's no surge protector or long extension cords.
Also, the inside wiring may zoned into different segment, I was restricted between two of the bedrooms and family room, not able to reach the 3rd bdrm nor the living room. I am not sure how to test it but it might be wise to find out how check that before you sink $200 into the adapters.