dedicated VHF/HF channel? = from 737 days, yes, Com 3 VHF and I believe on larger a/c, HF fallback
is engine-manufacturer's monitoring data a separate data stream, or multiplexed onto ACARS? How is data delivered to two interested entities on the ground?
Cannot help with all that. In my day I was told all ACARS comms were ultimately routed via Singapore, but that may well have changed.