The Matrix E5 5SSRM-02 is a variation on the SmartSwitch Router SSR-2-B128 (128 MB memory), which in turn is roughly equivalent to the X-Pedition XP-2400-256 (256 MB memory). Matrix E5 manuals are no longer available, but this set of XP manuals in our Legacy Products section might be generally useful: https://extranet.extremenetworks.com/downloads/Pages/X-Pedition.aspx, Documentation tab, Hardware Manuals -> X-Pedition 2400 Router.
Yes, that is correct. The 5Hs can switch among themselves, and this 5SSRM could optionally route among them if their direct connection backplane ports are disabled to force the 5SSRM as the sole intermediary.
Any smarttrunking port (equivalent to a LAG) would have been user-created, so look to the switch configurations for that. It should be on two or more front ports, since that's the only ports that could be trunked. The smarttrunking port may also be 802.1Q trunked so that it can serve as the single uplink port for all VLANs on the chassis.