Hi ceosol,
I don't know if what I've done is the same, but usually I modded the AndroidManifest.xml in Android / Template directory in Stencyl, and I've already try this method to write 26 as target version. I've compile it (but without sdk26) and the apk is working, tell me that target version is 26, but I know that this app isn't true api26.
So, do you think that your mod is the same as mine ?