Unfortunately I cannot answer your question on Jakob Elmer (b.1774) and Susanna Hefti (b.1781) as Glarus is not my area of research. I can just give a few general hints.
A huge genealogical collection (
Genealogienwerk Johann Jakob Kubly-Müller) is kept at the Glarus State Archive: it compiles information from all canton Glarus church records and other sources into family sheets with cross references. So - once you have found your earliest known ancestor, you wil be "lead through the family tree". The bad news is - you (or someone else) have (has) to visit the Archive - the collection is not online nor filmed e.g. by the Mormons.
The other possibility, of course, are the church records themselves - unfortunately they have not been filmed in canton Glarus either
.
Conclusion: if the information you find online is inconclusive or contradicting, I don't see much you can do - unless you can visit archives in the region.
There is no genealogical society for Glarus specifically, but several societies have members interested in Glarus - see
here: you could try and contact them, asking if anyone researches Elm families or visits the Glarus State Archive on a regular basis.