In my experience of big end fails, they don't do that unless totally 100% dry of oil which would be very odd if there is oil in the engine.
To be seized and unturnable, and with that noise, I really do think that a valve had dropped into a cylinder and that would only have been easy to fix if the engine was stopped when the noise first happened, but even then it would have been a head-off job.
Out of interest, maybe questions we should have asked in the beginning (haven't seen them in the thread, might have missed them if they're there)
did the noise start suddenly
how long had you been driving that day when it happened
how far did you drive it in total since the noise started