Just for refence with 10.11.9, if I put back the correct version number in BouncyCastle extensions, iText extension (and all my dependant ones) refuses to load at start time with the following error message:
[main] WARN ltInstalledExtensionRepository - Invalid extension [com.lowagie:itext/2.1.7] on namespace [wiki:xwiki] (InvalidExtensionException: Dependency [bouncycastle:bcmail-jdk14-138] is incompatible with the core extension [org.bouncycastle:bcmail-jdk15on/1.60])
Looks like I will have no clean solution on 10.11.9 
I will still continue testing with the odd version numbers on 10.11.9 but really think that upgrade time has arrived
For future readers, the installation way that seems to work is:
- before changing anything in xed files, install
org.bouncycastle:bctsp-jdk14version 1.46 on which iText 1.2.17 depends - change the numbers in xed files for
bcmail-jdk15on-1.60.xedandbcprov-jdk15on-1.60.xedforbouncycastleextensionfeatures and the main<version>bloc - find the
bctsp-jdk14.xedfile under the/data/extensions/repositoryfolder and add it an extensionfeature and a feature bloc mimicing the one frombcprov-jdk15on-1.60.xed. Do not forget to change the main<version>bloc too. - at that moment, install normally the docx4j-ImportXHTML extension
Definitely ugly because from now on, you cannot install anything depending on BouncyCastle, because in the end the system will choke with the inconsistent version numbers