The FEC overheads probably accounts for the redundant [for normal use] bits. I am not sure if the depth (D, as shown on most modems) has an impact. But R (for the redunancy in Reed Solomon code) should have a slight overhead with parity bits.
On ADSL (or at least BTW lines), any sort of interleaving always comes with a delay (minimum of 8ms). The delay being higher just increses the chance of data recovery.
There's probably a command for retrieving that information from your modem. I would like to search for it but doing almost anything on this laptop is awful. So right now, I don't want to get a headache from being frustrated by it.
Tone 122 would be 526.125 kHz, as tone 0 = 0 Hz (though, this is not utilised for data). The notch could be many things. Not sure what kind of radio would be operating on this frequency (and I do not want to go looking because of what I mentioned above in regards to this laptop). But
http://www.frequencyfinder.org.uk/ may provide some information. For the frequencies you've mentioned, you're going to want to look at AM.
I kinda miss having an ADSL line, even though it was awful. I wish I had taken more time when I had it to understand it much better. I am too scared to experiment with a VDSL line since there's a long way down in terms of sync speed!
