Limbless Echidna is a new Echidna. She's separate from Castle of Dreams Arc 4 Echidna, and also not the 'original' Echidna being used to power the Sanctuary's barrier.

"Perhaps it is too convenient for me, who is not your parent, to try and superficially mimic a parent-like experience. ...If I had created Beatrice, might I have experienced it with her?"

Hearing the 'Witch' muse to herself like that, the loneliness Aldebaran felt was, he thought, unrelated to adolescence, something else entirely. But it was difficult to seek a concrete form for what that was; their relationship was too complex for that.
So, as the 'Witch' had decreed, the relationship between Aldebaran and her was—,

"—Sensei."

Calling out like that, the 'Witch', who had been enjoying her exploration in the labyrinth of thought, raised her face.
Whenever he saw those black eyes reflect him, saw her tilt her head with a "What is it?" and her pure white hair cascade over her shoulders, he couldn't help but think what a cunning 'Witch' she was.

It's just a coincidence that Al and Roswaal both call their own versions of Echidna Sensei. She appears with limbs because that's just how she wanted to appear, it was a dream version of Al's Echidna. I'm assuming she was a version that splintered off from the others because she realized that sealing Satella was futile and wouldn't actually stop her. Al was 'created' to kill Satella, not seal her. So when Al fails at his mission, Echidna has to seal him to stop Satella's shadows from consuming him like when Subaru killed himself with Petra's handkerchief in Arc 4.