ElminsterAU

Yes, the read/write engine produces an in-memory data model with a usable API and it does not have any dependencies on commercial components. But I have not spend any time documenting it. As everything else, that core engine is written in Delphi. It's possible that it can be compiled with FPC (free pascal), but I've not attempted that. Using that core engine it should be relatively simple to write a program that loads and esp, makes modifications to it, and saves the esp again.

The full source of xEdit has not been released publicly. Neither under GPL nor any other license. Technically it is "Copyright by <me>. All rights reserved.". I did release the source for the core engine responsible for load/saving modules and representing/modifying them in memory under the MPL in the past with TES4Dump and FO3Dump. There hasn't been any noticeable interest in that source. Even if I would release the full source of xEdit, nobody would be easily able to compile it because it contains dependencies on a number of commercial components which I obviously can't redistribute in source. Furthermore, it is written in Delphi (XE by now) and most people are not familiar with that. I'm happy to make the full xEdit source (minus any commercial components which I can't redistribute) privately available on request on the condition that the source is not redistributed and have done so a number of times in the past. If anyone is interested in the source, please send me a PM with a description of what you intend to do with it.