Not having recovered the Halyard yet, I do not know the exact failure mode. However, considering there are several thousand miles on the halyard in some pretty rough conditions at times, and that I use overlong splice lengths, and that I sew up and parcel the throats of my eye splices, I doubt that the splice pulled out. I suspect that the line parted in the eye but, in any case, I shall report on my findings. FWIW, a knot in T-900 doesn't approach the strength of a splice (at least according to NERopes).
On Friday our current rigger extracted and re-reeved our main halyard after chopping off the damaged end and blown splice. The autopsy of the failed splice is now complete with photos of the dissected dead end following (I hope). The splice failed at the throat (as I had suspected). The "feathering" and discoloration of the outer most core strands compared with the clean ends of the ruptured inner core strands indicate the failure to have been progressive across the splice over some time, from outside to inside, as I had theorized. More diligent inspection of the splice on my part would have revealed the splice was failing but I was derelict with that tho' shall certainly not be again. A stress test of a length of the 10 year "old" halyard, just inboard of the failed splice, in the mechanics of materials lab at the local technical school Saturday morning by one of our neighbors, who is a professor there, indicated a rupture load of about 15000 lbs or roughly 4000 lbs above the rated strength of the line (10mm--11000 lbs) on the supposedly old, worn out, sun burned halyard. With that, Saturday afternoon I whipped the end of the halyard and connected it to the headboard shackle with a Buntline hitch. We took the yacht out and sailed her out to Egmont Key and back it 18 - 20 knots on an close reach, over sheeting the main all the way, and the yacht performed well (tho' that Buntline hitch will likely have to be cut off at some point in the future as there is probably no way of ever loosening it!).
Our current rigger, who was a close friend of our prior rigger who has since passed away agrees with him that a thimble is not needed with the new high tech lines. But. The proper way to use such splices is to make them rather longer than one would normally and to pass the entire eye through the shackle bail and loop it up and over the shackle so that both "legs" of the splice are carrying load as in a Cowhitch. I did that with our runners but not with our halyards. So. My next project is to chop the ends off our other halyards wherever I/we see feathering of the covers in the line, re-whip the lines and reconnect them with Buntline hitches. By end-for-ending the lines, I can probably get another 10 years out of them, assuming I live than much longer.
Hopefully, others can/will profit by my own misadventure. Snaps follow (fingers crossed!) FWIW...