[RPG] Can a Monk replace Elemental Attunement when learning new Elemental Disciplines

You are right, the RAW are contradictory in generally describing Elemental Disciplines as costing Ki and specifically the Elemental Attunement discipline not detailing any Ki cost. The following arguments apply:

1. A specific ability's description overrides a general description (though the risk here is that it can be a mistake) and the specific description here mentions no Ki cost where all the other discipline descriptions do;
2. The clear intent and power of the discipline is, as you have assumed, to give the monk the equivalent of the Prestidigitation, Druid-craft or Thaumaturgy cantrips;

These make it clear, in my opinion, that the Elemental Attunement discipline costs no Ki to cast.

It would have been better if the description had said something like "This costs no Ki to cast and is the equivalent power of a cantrip for other spellcasting classes" to make it clear.

The monk generates the effect, but not the element

The preceding sentence in the description of Elemental Attunement is "You can use your action to briefly control elemental forces nearby, causing one of the following effects of your choice:" so I would argue that the shower of sparks or the light mist is a product of the Monk directing the ambient elemental energies around him.

This may beg the questions:

1. Can you produce a fire effect if you're in the ocean?
2. Can you cast this spell when on a plane where the four elements don't exist?

There may exist a highly detailed answer for these questions but for the sake of keeping this answer brief and on-topic, it seems that you can always use this ability on the material plane, regardless of the apparent availability of elements locally, since the four elements are inherent there.

In other planes it may be down to the GM to decide whether elemental abilities no longer work, or whether the elements present in objects (and bodies) from the material plane count as "elemental forces nearby"