I use Dropbox for the catalog as well as for the original photos (two computers, 20 minutes drive apart - home and studio), but I am the only user.
I have had absolutely no issues with this setup, but it's crucial to let Dropbox finish syncing before you launch Lightroom, and not to turn off the computer until Dropbox has finished syncing. (this of course applies to both of you)
And both of you working with the same catalog at the same time would most likely cause problems.
I have no experience with smart previews, but I can't see that it should represent any problems, apart from the need for them to upload/download before any action is taken.
Some comments added to your process, which is possible-
Here is the process that I would like to do:
1- Create new catalog with just one wedding with smart previews
You can create a new catalog, OR better- Import the wedding photos to one folder in your master catalog library, then select the wedding folder in your 'master' catalog and do "Export as Catalog" including Smart previews only.
2- Upload the Catalog and the Smart Preview file to some where online (currently we use dropbox, but I am open to any file sharing service)
Yes, upload the complete folder created by "Export as catalog" containing both the 'Exported Catalog' and the smart previews.
3- She downloads the catalog and smart preview to her computer, does edits
Yes, the folder containing both catalog & SPs
4- She sends me back the catalog, I download and import the photos back to my main catalog.
No, not exactly- all you need now is to download the edited Catalog, and working from your 'Master catalog' do "Import from another catalog". This will bring the edits back into the 'Master catalog' and update your previews with the edits.
And yes- as above!
it's crucial to let Dropbox finish syncing