Indeed, to avoid starving the heat pump of water flow (which also drives up the flow temp if you hit minimum output) you really want to try and set the downstairs TRV’s to the maximum setting (or remove the head entirely) - similar to what you’ve done.
Any differences in heat output/room temperature can be dialled in through balancing (assuming the radiators are generally able to meet the peak heat demand for the room)
It looks that way - assuming you have a heat curve defined? I’ve found going lower than sat 32C or so doesn’t really work that well from an efficiency perspective on my system. YMMV.
Switching to LWT mode and the 4C overshoot may result in some overheating, but you want to then reduce the weather curve (but not too far) - and it should allow balancing to be done if you can get to a steady state.
If rooms are not getting to temperature, ensure the valves (not the TRV as that should already be at max) are fully open.
Any rooms that get hot too quickly or overheat, start closing the valve little by little.
Unfortunately, most valves only start restricting flow rate at a certain point, and then they are very agressive, so you can turn and turn with nothing happening, and then all of a sudden the flow stops…its at that point you’ve found the “authority”, and then its’ trial and error:
room too cold, open slightly.
room too hot, close slightly.
There is a thread specifically on balancing here as well…: