There are some functions in the C API that were renamed ~ the 2017 release (with the old copies / aliases still present for backward compatibility. The note in the "version.txt" file said that the old named functions would be removed in the 2019 release.)
In addition to those, is that new functions get added in newer releases. You cannot call newer functions if running as a “live” extension in older versions.
I state the obvious because on Windows I believe error 127 is ERROR_PROC_NOT_FOUND.
(Ie, The specified procedure could not be found.)
Not that I know of. I’ve tried modifying the download URL to the 2017 release name of the zip, but it no longer works. Apparently only the download link for the latest release works.
For this reason, I myself kept SDK packages for Windows all the way back to 2014 (14.1.1282) mostly for the docs and samples. The packages are too large to post as forum attachments, and I don’t have distribution permission.
Perhaps one of the distributers like @SketchUp3D_de (Posh GmbH) can get it for you ?
Have you read Dale’s SDK thread … “Best practice to support multiple SketchUp versions?”
The other main difference (if making any Ruby C API calls) is that 2017 uses Ruby 2.2.4 and SketchUp 2019 is now up to Ruby 2.5.5. (But 2018 and 2017 both use the same Ruby version.)