The 12:56 time was not implied as shortly before 1:07. The Preliminary Report clearly implies a number of routine messages for an appreciable time prior to the last message prior to the loss of transponder contact. The quote is directly from
http://www.dca.gov.my/MH370/Preliminary%20Report.pdf.
I was focused on the 'last ACARS message time' which matches exactly to the second with the last Satcom message (prior to the pings, missed calls, and two logons post IGARI). And which, I believe, establishes clearly that the Satcom message is from the ACARS system and that MH370 was using Satcom and not VDL to transmit its ACARS messages.
I have no idea to what the 12:58 time refers. It may be when the aircraft was first powered up that day (i.e. an earlier flight) or it may be some time near the Satcom initial login of 00:13 but documented incorrectly, or it could be some entirely different event that the preliminary investigators considered relevant in this context.