The trouble is, there is a major difference in the amount of detail Minfilia went into when describing the Echo and how it functions between 1.0 and ARR - in 1.0 she went into great detail about it, which is probably how the player is able to gain control over it and use it seemingly at will (she even asks the player to use it to demonstrate to her that the player actually possess it). In ARR however, she explains it briefly, then handwaves it away as she goes back to talking about the Primals.
Unfortunately, the actual use of the Echo in ARR seems to be more re-purposed as a convenient explanation for how the player is able to avoid the corrupting influence of a Primal (hence making them a valuable commodity), which was nothing more than a side effect the player discovered by accident in 1.0 - the two most important features of the power: being able to view another's memories as if the viewer themselves had actually experienced it first hand, and also understanding any language in the world even if they had never spoken it before (xenoglossia), are in ARR only occasionally mentioned/shown and virtually forgotten respectively.
Which is kind of a contradiction, given the fact that in ARR the Archons themselves state to the player that the player seems to be the most potent user of the Echo they have ever seen, so being able to control that power at will would kind of be expected (hence when Minfilia lamented that "if only we could use that power whenever we desired" - paraphrased, I was a little surprised, because it directly contradicted her own actions and words to the player on the subject in 1.0.)
There was also a lack of consistency in the Echo's power as well between members, in 1.0 some, like Minfilia, stated they could only experience past events from memories and couldn't understand other languages, while others couldn't view memories, but had a remarkable understanding of language - again in 1.0 one such npc was a lalafell who stated he had actually used his meager Echo power in business (he didn't elaborate on exactly how he used it, just it was "profitable"). And of course, some, like the player, had mastery of both aspects.
In any event, as for your first question, 1.0 stated that the amount of people 'awakening' to the Echo's power seemed to be increasing (Minfilia herself complained that the 'mere trickle of reports' of new prospective Echo users had grown into 'a veritable mountain', and mused at who, or what, was causing this sudden increase in Echo users. At that time it should be noted that the exact source of the Echo was unknown, and Minfilia and her colleagues at the time believed it was a member of the Twelve responsible (hence her Echo organization was called 'The Path of the Twelve') - of course ARR has virtually confirmed it was
Hydaelyn/the Mothercrystal, something that was hinted at briefly in 1.0 but never actually mentioned.
. The Path in 1.0 was shown to be a fairly large organization with members from all walks of life, of different races and nationalities, and ages as well. One Echo user was a hapless lalafell girl who was stated as being noble born but had been thrown out on the street by her parents when she started having 'visions'.
By the time of ARR though, the amount of Echo users who stayed on with the Path's new successor organization, the Scions of the Seventh Dawn, seemed to have reduced considerably. Whether the missing had been lost in the Calamity, had gone into hiding, or just left the new organization, is unknown. But it's highly likely there would still be people being awoken to the Echo even post-Calamity - new players being one of them.