Hi,
For your first question, you will want to use Aliasing. Aliasing allows you to create one-time use names/links to each of your streams. Each time a user goes their page you can set a new alias on the EMS, allowing the user access to the stream. There are also other ways you could use aliasing to achieve this user "sandboxing", perhaps you can describe your environment a bit more.
You can certainly push a stream from FFMPEG into the EMS. I would recommend using RTMP for this, although RTSP will also work.
If I understand your third question correctly, the EMS can create HLS, and that HLS can be used to do stream seeking (jump forward, jump backwards)