Due to some privacy concerns by some part of my family I am maintaining the "complete" family tree and archive on a site run on my computer, however I would like to continue getting information from and contributing to Geni for other parts of the family with no such concerns.
I was wondering if there is any way to import and export a subset of records, eg.let's say descendents of X only, or I can only do this manually, if I do a full export and filter out the recordsI don't need manually.
A GEDCOM import on Geni should only be done once for a person, if you do it a second time you will create a duplicate. So it is not a good way if you want to update Geni more than once for the same profile.
As far as I know there is no easy way to synchronize a local copy with Geni.
Following on this topic. I want to EXPORT my complete family using GEDCOM. I assume this is possible?
That is not so simple to do.
If you are connected to the Geni Word Tree your tree would have about 180 million records. The max. number of profiles you can export is 50000 records if you are a Pro user and 5000 if you are a Basic user.
You can export from any profile you added to the tree, so you could do multiple exports and combine them. You have to take into account that an export will export any tree and data conflicts and will export an inconsistent GEDCOM (because of the export limit)
If you do multiple export you also have to take into account that Geni will have been updated between those exports.
see also How can I export my GEDCOM?
You can also look at https://help.geni.com/hc/en-us/search?utf8=%E2%9C%93&query=gedcom+export to see some additional info.
