Copyright for Zemirot

There is a well known siddur which is going through a second edition, which I and others are editing. In the new edition
he is planning to expand the collection of Zemirot for Shabbos.
There is a bentcher put out by two avreichim which has been produced ע”פ כתבי יד ודפוסים שונים.
The question is: I bought their bentcher. May I incorporate their findings into the new Sidur?
They spent years uncovering this information from many manuscripts.
Is that information theirs, or does it belong to those who authored the original manuscripts?
Perhaps the author of the Sidur may use their work as long as he does not declare that the Zemiros are edited
on the basis of Kisvei Yad and thus profit (assuming he is making a profit) from the authority that this
lends to the Sidur. If he gives no indication to that fact at all, and just edits the Zemiros according to
their work for accuracy’s sake, is there an איסור in that?

Answer:

This is a complicated copyright question, which is in some senses similar to the original copyright issues of brought in the case of Rabbi Hildesheimer’s Machzon (to which the Chasam Sofer and other luminaries related). The general leaning is towards stringency in this matter, based both on formal halachic principles, on the law of the land, and on the ethical issue.

I certainly think that the best option is simply to get in touch with the publishers of the Zemiros and ask them for permission to use the corrected version, perhaps mentioning that the version is republished with kind permission of the authors/editors.

Best wishes.