Yes it should do, server 10.0 onwards.
I use domains from a sql database in an editing appliation built with the flex api. Have a look at the REST API help here:
As far as I am aware yes, but I thought I had read somewhere that Esri had changed the way attachments are stored within AGOL so that they are stored outside the feature class so you do not get charged for hosting large file size feature classes that are holding images. But I cannot seem to find that info anywhere, maybe someone from Esri can ellaborate.
some info on setting up the hosted feature service for attachments can be found here: