XLIFF and potentially XML:TM are both critical to maintaining control over your translations and avoiding unnecessary costs. Simply sending content through Trados is the least effective method you could pursue, since you will be at the mercy of the translation memory (errors, lack of updates, etc.).
As the chair of the OASIS DITA Translation Subcommittee, I can assure you that the list of items you've created in your email is the right one to pursue. Whether or not Windchill supports any of this—I really couldn't say. If you can export a DITA map (if you're using DITA), you can convert the map to XLIFF and send one file to your LSP, rather than many. XLIFF should also work fine with DocBook files. Do read the MaxPrograms article about the process. We're getting ready to republish Rodolfo Raya's article through the DITA Adoption Technical Committee soon so it's an excellent source.
In the best case, you would want to import your translated files, after you've recovered them from XLIFF, into your CMS. However, in the best case, you want those files to be linked to your source files. In XML CMSs, each source file is linked to each translated file. Therefore, if the source file changes, you will know which target language files need to be retranslated. I prefer to send only the changed files again, not everything. Will Windchill help you with this? Better ask someone who knows how they handle translations.
Let me know if you'd like to discuss this further.
JoAnn
JoAnn T. Hackos, PhD
President
Comtech Services, Inc.
303-232-7586
Joann.hackos@comtech-serv.com