i don't understand much of the chemistry involved but i thought that if Ca and Mg (and possibly K) didn't appear at a certain ratio, they would be antagonistic and unavailable.
Can't just say "yes". There are bio factors and chemical / ion exchange factors.
From the standpoint of ion exchange factors in potting mixes, there is a priority of selectivity. Generally the potting mixes want to take in the monovalents and give up the divalents. In order to drive it the other way you need to have more divalents available. If you present a 50/50 mix of divalents and monovalents, there will be a selection to take up the monovalents until the concentration of divalents is high enough to produce enough ionic or charge "pull" on the media to balance or reverse the selection process.
From the plant bio process there are two (at least two) basic systems, which are not mutually exclusive. One is ion uptake and the other is various metabolic processes. The requirements for correct ion ratios to avoid antagonism is probably more important from the standpoint of intracellular metabolic requirements. Ion uptake can be both passive and active, which means that environmental ratios of ions can be overcome for specific metabolic needs.
In the environment available K is relatively rare, especially for orchids living on limestone cliffs and up in trees. But if you analyze leaf content for minerals, there is more K than either Ca or Mg. If you look at marine kelp, it has more tissue K than Ca and Mg, but sea water has more Mg than Ca and K. K is actively selected and pumped into plant cells from inherently low concentrations (especially compared to the normal environmental amounts of Ca and Mg). My present hypothesis is that orchids, coming from nutrient impoverished environments, are very efficient at K uptake, but do not have the metabolic hardware to deal with excesses (i.e they can't turn off the pumps easily). And over the course of a year using balanced fertilizer mixes in bark or CHC substrates, you can quickly end up providing an environment with an excess of available K.
Most fertilizers were developed based on testing of common agriculture plants (which are very fast growing compared to orchids) and are harvested to varying extents. Looking at the agriculture litterature its interesting to note that the K consumption (not leaf concentration) of corn (temperate plant completely harvested to the ground every year) is many times greater than for coffee (a tropical plant with only the fruit harvested regularly), but if you analyze tissue, the concentration of K is about the same for the 2 plants.
Corn and wheat will reduce soil K to extremely low levels so during the growth season you need to add a ton of K, but coffee growers don't throw nearly as much K to their plants as corn growers do. And I think orchid growers should use even less.
Orchids in generally grow even slower than coffee and generally don't have a harvestable commodity (except for periodic big floral spikes). The big multis can put a lot of K into new tissue growth during the growing season, but smaller compact species probably can't handle nearly as much. There's not much growth in the winter, but how much do you cut back fertilizer in the winter, building up K in your potting mix?
This post is really starting to ramble, but my short answer is that no, the uptake of nutrients is not passive and even from a solution of ions at set ratios.