Well, the thing is, I think we need to finally let go of the concept of the Echo being the 'past' - everything that has been stated (in 1.0 and by Ferne) shows pretty conclusively that it's not, at least, not the literal, physical past. Lets not mince words - viewing the Echo is nothing but a highly lucid day dream - the player is seeing themselves projected into someone else's memories, hence the explanation for all the weird things that happen to the player during the Echo.

The player is not stepping literally back in time, their body is remaining physically in the 'present' while their mind simply views someone else's memories from an earlier time - this is what Minfilla tells us and, when you let go of the idea it is literal time-travelling, does make perfect sense- why else did the elezen say to Derplander in the original opening "Look who's returned from his reverie?" when he jumps out of the Echo? He was still there in the Adventurer's Guild in Limsa Lominsa during his little sojurn to Mor Dhona, just utterly unresponsive and entranced while holding the guildleve.

Hence, in my opinion, the 'Echo within an Echo' is simply an extension of this - it's just how the Echo viewed a person's memories - in most likelihood it was jumping from one person's memories to another (this would also explain why in the Gridanian one how Yda and Papalymo came along - the player jumped into one of their memories to do so). And ultimately, all that Echo jumping probably caused such immense strain on the player's mind/brain, it forcibly ejected them from the Echo completely as a 'fail safe' to prevent further harm to them (hence why they collapse within it and then wake up back at the Inn Room).

Or maybe I'm grasping at straws here.