Would I be correct in the assumption that all three modes can be on fast path if the connection was good enough, and that the DLM would use differing trigger levels of errors for applying/increasing interleaving?
So the speed profile will require the most errors before the DLM intervened and with the stable profile the DLM would intervene earlier, the other would be in between.
Exactly right. The different profiles offer different thresholds before DLM intervenes, though it monitors for resyncs as well as for errors. If your line is so perfect that it never resyncs, and never has any errors, then it can be considered stable.
My old ADSL1 line used to get less than 10 CRC errors per hour. Why would you need to take fastpath off that just to give it a label of "stable"?
I wonder what G.INP will bring, when BT get a different tool to use instead of FEC/interleaving (or to use before they revert to FEC/interleaving). My little reading on that subject suggests that FEC and re-transmission are used instead.