Comment 15 for bug 264912

Even on 2017-06-05, I cannot find other pages about how to do this. Even Select All In All Layers does not have anything like a complementary Paste All In All Layers. But I've just discovered, in Inkscape 0.91, that the Import feature can be used as a workaround instead. It works for two separate SVG files, each with many layers and sub-layers.
If one file is LastPart.svg and the other is NearlyFinished.svg, first close LastPart.svg (to reduce memory use and confusion), open NearlyFinished.svg, choose File, then Import, then choose LastPart.svg. All of LastPart's layers should appear, although probably not in the same position on the original page, and LastPart's layer's names appear at the top of the Layers list. On my average PC, it took several seconds with a 34K LastPart.svg and 532K NearlyFinished.svg file. These layers can be dragged to re-arrange like others. This process can be undone and re-done. If instead you first select (highlight) one of the NearlyFinished layers in the list just before importing, all of LastPart's layers and sub-layers should load within that selected layer. The import allows multiple layers with the same name, as Inkscape does, which can get confusing. I'm only a beginner, and have not experimented much, but nodes, fills, opacities, names, and visibilities seem to be preserved.