This sort of extensive editing would certainly create a new edition and therefore a new copyright under US law.

Not since Feist v. Rural. Many (in fact most) federal courts used to use this "sweat of the brow" standard to determine what was subject to copyright, but that was very explicitly overturned by Feist. The supreme court clarified that it doesn't matter how much work went into producing something; it is the act of creative expression that creates copyright. If the work isn't "creative" then it can't be copyrighted. Some edited classic works may still pass the Feist test if the edits include making editorial decisions, but an earnest attempt to reproduce an old work as accurately as possible is the opposite of creativity.

Now of course scholars will continue to claim copyrights on edited classic works, and they will probably sue infringers and may even win a case or two. They'll lose eventually on appeal, but it may take time and court costs to get there. 0