My guess is that the reason there has been no response is that no one has any more information than is in the database. Given that its first appearance in the database is in 1940, in the Broadman Hymnal, I think it not imlausible that it may be a folk attribution sort of like ascribing "Away in a Manger" to Martin Luther, or taking Lowell Mason's word for it that he got ANTIOCH from Handel. I also note that most of the the hymnals that have a high standard of scholarly backing to their attributions say "Arr[anged] from" Mendelssohn rather than "[Composed] by" him, it may be that the they had found something in Mendelssohn that might possibly have led to this tune. I also note the additional attribution in the Hymns of the Saints (1982) to William H. Callcott. If you are still really worried about this particular attribution, and have not yet tried this, you might do well to look through Callcott's published arrangements, as he may have specified his precise source. After all of this, if you still have not located a specific source in Mendelssohn, I would suggest crediting it as "Attr[ibuted] to" Mendelssohn, as this does appear to be the overwhelming consensus among past hymnal compilers, even though you can't find the original autograph to corroborate their assertion.
As another person who plans to publish hymnals in the future, and who actively collects hymnals, I'd be interested to know details of your plans, and to be notified when they come to fruition. And I thank you for indirectly bringing Callcott to my attention, since his ELIM which I glanced at more or less by chance, is a tune I may have a use for in my forthcoming tunebooks. Or even two uses for it in two separate tunebooks.
We were not able to reproduce this problem. It is possible it is fixed now. If not, please let us know. If possible, send a screenshot.
I just found it on the wiki, here is my email address: firstname.lastname@example.org, I will email you if I get yours back of course :)
There is some training involved. Information and training materials can be found in our wiki - www.hymnary.org/wiki. I can give you more information if you email me.
This is a minor bug. The spreadsheet should upload with all the information so there is no need for concern. We do have plans to fix the bug in the future.