You can certainly engineer a solution which uses a web front-end to do all these things,
but you'll be making extensive use of ArcObjects (which is not, as you assert, equivalent
to the Desktop application -- it is the code which which implements geodatabases).
There is a tremendous level of effort to accomplish this goal, however, and it will serve
you well if don't think of this as an "sde" thing -- This is a geodatabase application, pure
and simple, and ArcSDE would only play a tiny part in the overall implementation.
- V