The 1979 compilation/soundtrack album The Kids Are Alright has a 2'45 abridged alternate mix of this song, as heard in the September 1967 mimed performance on The Smothers Brothers Comedy Hour.
Would you say the meaning would change if one took out the comma?
And do these work:
3) The 1979 compilation/soundtrack album The Kids Are Alright has the 2'45 abridged alternate mix of this song, as heard in the September 1967 mimed performance on The Smothers Brothers Comedy Hour.
Yes, the comma makes the clause nonrestrictive; it's absence, restrictive. Your versions #3 and #4 signify as your state.
If you were writing in AmE, you could make this even clearer by writing either 3) "the alternate mix, which was heard" or 4) "the alternate mix that was heard."