OMG loook here... a capillary tube. What a problem! OH NO!!!!!
Not.
These mini-split units don't need a metering device outside the box. You aren't going to kill your unit running in reverse. Let me explain...
These units are made to withstand lots of torture from the outside world. All the working parts are inside the box. The indoor heat exchanger can be whatever you like. As long as there's not much pressure drop (from your readings I can tell there's not) and the indoor unit will actually exchange some heat (it had better), it will work just fine. The capillary tube doesn't care at all, it will work in both directions and do its job just fine.
If you look at your handwritten diagram, The expansion device is a cap tube, a meter or two length section of 1-1.5 mm ID tubing. It acts just as a resistor does with electricity. The more luxurious models have a check valve that shunts a small portion of this resistance in cooling mode. With a shorter cap tube, there is less resistance, so you have more mass flow (current) at a higher evap temperature. In heating mode, the extra resistance gives you a lower ultimate evaporator temperature (for better heat gathering in winter) at the cost of less heat transfer. This is irrelevant at low temperatures, since flow is naturally pinched off by low suction pressure at the compressor intake.
At freezing temp of water, the r410 evaporates around 100 psig and gains nothing more in the evaporator due to low ambient temp. At 25 degC ambient, evaporator pressure is roughly double that at 200+ psig. Being a constant displacement device, the compressor can only pump what it is sourced by the evaporator pressure. The cap tube is a relatively linear device, and it doesn't care whether it has gas or liquid flowing through it. If your system is undercharged, the condenser will run out of liquid at some point and feed the cap tube gaseous refrigerant. The cap tube will pass much less overall mass through it, and evaporator pressure will drop, limiting flow through the compressor. The main idea is that cap tube systems follow evaporator discharge temperature.
As for charging the system, these units are critically charged. A very few test conditions are chosen to be godly and everything in the unit is designed around these few conditions. Weeks and months of trials are run, and an optimum refrigerant charge is determined. This value is printed boldly on the ID plate of the unit. Any and all reputable service techs and engineers will tell you that all mini-split units can only be charged by weight of refrigerant, period. I will tell you that unless you really modify your outdoor unit, you aren't going to get much better performance than what you get by charging by weight. Your optimum charge will be within a few grams of the charge on the faceplate.
There is a way to measure superheat with your system the way it is. In cooling mode, you take a pressure reading on the big line (evaporator discharge-->reversing valve-->compressor suction) and compare this pressure to its equilibrium temp on a PT chart for r410a. If you're lucky, your gauge will have a scale built in. You then take temperature readings along the evaporator discharge line and subtract the PT chart value from these readings. Unless your tubing is super-insulated, the cold refrigerant will draw heat through the tube walls as it travels towards the compressor. This temperature gain (sensible) above saturated suction temperature (from PT chart) is the superheat.
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