A high pH is needed for RUBISCO to function because of the organic chemistry it is catalyzing. The reactions take place only under a low proton concentration.
It is very dangerous generalizing from experiment to experiment. Plants are very diverse, biochemically speaking, and reactions that are occurring in one plant may not be occurring in another species. This is even more the case when it comes to cyanobacteria which are billions of years removed from plants.
Also, plants have a plethora of ion channels. Some are run by ion exchange (one ion in, another out), these are antiports. Then there are synports where two ions are imported together. These can be a cation/cation, cation/anion or anion/anion. There are also channels/pumps that are run on ATP or NADH supplied energy. Some pumps/channels are doing the same job but are expressed in different locations and are also expressed by different genes. The whole story gets very complicated fast as one genus may use pump A in the chloroplast while genus B won't have that pump but a different one doing a similar reaction. We can't generalize about plant metabolism when it comes to the specifics.
What does hold is the physics governing these pumps. This described by the Nernst Equation (taken from Wiki):
. As you can see the major effectors are the internal/external ion concentrations, the difference in charge between the membranes and the permeability of the membrane to the ions. If the movement can't occur spontaneously, then a power source is needed to drive the movement. If you keep flooding the cell with K+ then the membrane will be less permeable to NH4+ and Ca2+ etc... and more power will be needed to import these against the electrochemical gradient. This holes equally true if there is an excess of Ca2+ or NH4+ inside the cell which would make it hard to import K+ etc...
How much the plants can vary the relative pump/channel concentrations in the membrane to transport specific ions is, in the case of Paphs, an unknown. If they can decrease K-channels and increase Ca-channels then the effect of K concentration may be of little importance... Each plant will have evolved to either cope with the change in ions in its environment by evolving the specificity of each ion to the channel or the amount of channels/available; or the plant (unable to evolve specificity or pump quantity) simply survives in a specialized niche which is just right for its delicate biochemistry. Generally, living organisms maintain their enzymes/pumps/channels etc... operating very far from the equilibrium point so organisms have a lot of flexibility to adapt to changes in the external or internal environment without bothering about changes in gene expression. I don't think we have any idea just how flexible our plants are... but the various medium and fertilizer experiment suggest that there is a range and it is not very robust.
Whether the issue is absolute K or NH4+ concentration, or how these concentrations affect the metabolism of other nutrients is very difficult to pry apart without large, well thought-out experiments. In the case of orchids these would be very expensive experiments as the plants are no as easy to work with as the normal experimental model plants that grow easily and whose roots and leaves are readily available for analysis.
I think the best we can say is whether the fertilizer/substrate method works or not. How it works will remain a mystery until you get an orchid growing president in the white house to set up the National Orchid Research Institute funded lavishly with tax-payer dollars to benefit Dutch commercial orchid producers.