I haven't read the entire thread, but the earliest possible reference I can find to the hymn is in an ad in The [Philadelphia] Press, Jan. 5, 1859:

"SAY, BROTHERS, WILL YOU MEET US?--THE SUNDAY-SCHOOL BANNER for January, Part 8d, containing the Music and Words of this beautiful Union Hymn, is now ready. Price at the counter, One Cent per Copy or 50 Cents per Hundred.

"Published and for sale by

"THE AMERICAN SUNDAY-SCHOOL UNION
                                        1122 CHESTNUT Street"

The hymn was apparently new. John Brown's raid on Harper's Ferry took place ten months later, on Oct. 17-18. he was executed on Dec. 2.

San Francisco Evening Bulletin, June 26, 1862:

"ANNIVERSARY OF THE SAN FRANCISCO SUNDAY SCHOOL UNION

"...between 1,000 and 2,000 children of the various Sunday Schools...

"When the report ended, the girls sang to the tune of 'John Brown':

    Say, brothers, will you meet us,
    Say, brothers, will you meet us
    Say, brothers, will you meet us,
       In Canaan's happy shore.

and the boys replied:

    By the grace of God we'll meet you,
    By the grace of God we'll meet you,
    By the grace of God we'll meet you,
       Where parting is no more.

then all in full chorus:

    Glory, glory, hallelujah,
    Glory, glory, hallelujah,
    Glory, glory, hallelujah,
         For ever, evermore.

"Although to tell the truth, the majority of voices drowned the 'For ever, evermore,' with 'Our souls are a-marching on.'"