You left out the most important meaning of psallo in regards to this verse: in the NT to sing a hymn, to celebrate the praises of God in song. At the time in which the NT was written, the word had taken on a meaning of purely singing a hymn.
Also, in the definition of psalmos, let's look at the part that you had in bold:
accompanied with the voice, harp or other instrument.
Notice it is possible to just be sung with the voice. A harp or other instrument are other options, but are not absolutely implied in this term.
Then we would be in complete agreement. Musical Instruments are OPTIONAL!
I'm just so glad we could end this discussion on this agreeabe note. (no pun intended).
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Marlowe