Radiant heating with metal rods

Thermal mass does not block heat; it stores and releases heat, and it slows down the flow of heat. It will absorb heat when it is colder than the surrounding area, and it will release heat when it is warmer than the surrounding area. These characteristics are useful in the following circumstances:

1. A thermal mass wall in a climate/season where the outdoor temperatures cycle above and below the indoor comfort point. The heavy walls will slow down the flow of heat so much that by the time it reaches the interior, the outside temperature will have dropped, reversing the flow of heat out of the wall. In order for this to work, the wall must be very dense and very thick; 12" thick rammed earth or adobe, for example.

2. A thermal mass element within a building's insulated envelope. Such a thing can be used to store free heat generated from sunlight, cooling, electricity usage, etc and store them until the sun goes down and everybody is asleep, whereupon it will release it, moderating the interior temperature and reducing its daily fluctuations, especially at night.

Insulating the underside of the floor is a great idea, as it will make your thermal mass floor fall into category 2.

Preventing the sun from hitting your walls or windows is likewise always a good idea; heat gain is driven almost entirely by direct solar gain.

Keeping the garage door closed may or may not help; try it and see.

1. No. You may risk damaging your unit by running it in cooling mode when it is too cold out (although some systems have safeties around this), but heating in low temperatures should not damage the system.
2. No. A correctly functioning heat pump should not ever drop below 100% efficiency.
3. This is dependent upon the specific heat pump model. Often, manufacturers provide charts/graphs describing the efficiency at different temperatures, but I do not see one available for yours. Two data points are specified; at 47f it is rated at a COP of 3.5 (or 350% electrical efficiency), and 17F a COP of 2.42.
4. No, but the heating capacity is often close, in mild weather. Yours is rated at 33,400 BTU/h at 47f and 21,400 BTU/h at 17f. Measuring this requires two temperature sensors (before and after the coil) and an accurate airflow measurement.
5. Your system defrosts on a timer, after 30/60/90 minutes of compressor run time, depending on a jumper setting. (this is less efficient than smarter systems that detect increased air pressure from from frost blocking air movement through the coils).
6. Protecting the unit from falling ice from eaves, and from accumulation of snow, is a good idea. However, make sure to leave ample space so that the airflow isn't restricted.
7. It says on the side that your blower draws up to 2.3 amps.