where do I start? panel write is used for the beginning of the song, if you use it at the end you'll overwrite the immediate previous changes you just made, simple as that.
On the 128 polyphonic instruments you have a choice of having channels 1, 2 and 3 as R1, R2 and L to play directly from the Conductor or as parts 1, 2 and 3, it does not affect your editing at all.
Track assign is important for the reasons in a previous answer to Heather, smf is different to technics mode. There are 16 tracks in the sequencer but 24 parts to assign for recording, and 29 for midi output.