Question #2. What specifically needs to be done?I understand the concept of "more air" or get the static pressure to 0.5" WC, but I want to know specifically what the HVAC professional needs to do to achieve this, e.g. enlarge cut if for a 20" x 25" filter, add another cut in for two filters (perhaps by wrapping the return duct around the furnace so I can have a filter on the side AND the back, enlarge the main return trunk, replace the high-limit switch, etc.

We can only guess from here. Since we don't can't see and test things for ourselves. Your HVAC pro is the one that can tell you what needs done.