"including inspection panel" means that when inspecting the wings you should also access any inspection panels - looking up an online manual for the DA20 describes thusly:
The bottom shell of both wings has holes for access panels below the flap and aileron bellcranks. The bottom shell has a hole for an access panel below the B-bolt. This gives access to the B-bolt for removing the wings.
Which means you should probably open any inspection panels which are accessible without tools (ie, a screwdriver) and check the general condition of any parts inside, such as structural spars, linkages, bolts etc.
The term "Free" in a checklist against anything such as a knob or lever means that that lever is not impeded from movement in any way and can travel the full range of expected movement. For example, that there is not a control lock in place, or something in the way of its movement or that any bearing have not seized it in place.
The ones which say "Free, OFF" probably have a reason for doing so, the ones that say just "Free" probably sets them in a latter step to something sensible. eg, "Trimmer: Free" - later on pre-takeoff it will say "Trimmer: Set take-off". Contrast to "Mixture: Free, ICO" - its important that mixture is reset to Idle Cut Off after the check for "free"