Looking at your diagrams and readings, I can tell your system is doing well. Since it is in heating mode, we can only get pressure readings from the high pressure side of the system. The valves you have installed are on the same side of the system... the 3/8 line is compressor discharge (gas) line and the 1/4 line is the liquid line. The 3/8 line is at 375 psig, which corresponds to 60 degC saturated condenser temp, at 68.9 degC you have 8.9 degrees of superheat (not the superheat that matters...). The 1/4 line is at 350 psig, which translates to 55 degC saturation temp, and at 32.6 degC the liquid refrigerant has subcooled 22.4 degC below its condensing temperature!
What this means your indoor heat exchanger is doing quite an awesome job at extracting heat. The approach temperature is only .5 degrees, so you could definitely slow your flow on the heating water and not lose hardly any heat transfer. This slower flow would yield you up to 15 degrees hotter water if you need or want it.
If you want to check superheat on the indoor side of your system, you will need to run the unit in cooling mode. In this case, the 1/4 line will have saturated liquid/vapor mixture running through it and the 3/8 line will have superheated vapor flowing through it. You would take your superheat reading from the 3/8 line, but from the effectiveness your heat exchanger is showing during heating mode, you will most likely have another close hx approach and gobs of superheat present. The bottleneck in the system is definitely not your indoor hx.
To measure superheat of the outdoor side of your system, you would need to install a service valve in the line between the outdoor hx and the reversing valve. If your unit was charged by weight, it will do its job just fine. If you haven't messed with the control board inside the outdoor unit, it might even have its own defrost control built in. It would be foolish for the manufacturer to include a reversing valve if the unit could not defrost itself if it needed to.
Also, you can measure the heat transfer of your unit the way it is. If you know the flow rate of the water through your hx, you can calculate instantaneous heat transfer. If not, you will need to know the volume of your heat store tank and estimate the volume of the plumbing running between it and your hx. Heat transfer of the water side is all sensible, so it is easy to set up a time trial of your loop as long as you don't use any of the heat generated during the trial.
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