RHUB (http://www.rhubcom.com/) should be on the list.
1. As single click as possible for the remote end user needing support
2. No firewall configuration
3. Several people could login to the same computer to provide support
You may consider RHUB (http://www.rhubcom.com/). It is a plug-and-play appliance-based web conferencing solution regardless of platforms. It comes with 4-way video. Compared to subscription service, it is a 3-10 month pay back purchase.