1) sync assets to FileServiceProvider? seems only JS and CSS work
If you upload a file using the Broadleaf admin for media assets, that uses the FileServiceProvider and thus stores everything on S3. In Broadleaf 5.2 with the import module
(a commercial module) it does a bulk upload of assets which will then put them in the right spot for S3. Without that, as long as you go through the BroadleafFileService interface to load files then that will go back and forth to S3.
2) configure UI to load static assets directly from CDN?
Check out the Broadleaf CDN configuration docs
The Broadleaf forums are being retired as a readonly archive of questions. For active discussions and questions, check out the broadleaf-commerce tag on Stack Overflow
which is actively monitored by the Broadleaf team.