[da_chicken]

You're close.

The Welsh or Icelandic "ll" is not quite the same. That's a "voiceless lateral fricative", lacking the alveolar break that earned it the "t" in "tl" for the Latinized spelling. It's much closer than most languages get, but it is a different sound.

The Nahuatl consonant is a "voiceless alveolar lateral affricate". It is a single constant represented with [tɬ] or, more correctly, with a tie bar between those two glyphs: [t͡ɬ].

[dav_Oz]

I stand corrected you are right there is no isolated use of [ɬ] in nahuatl as a phoneme it is used only in the context of an affricative /t͡ɬ/ I got ahead of myself in trying to isolate the sound [ɬ] for untrained ears.

To get back to the original point though if I'm not mistaken again in standard mexican spanish /ʃ/ as a phoneme is lost entirely and only appears in the affricative /t͡ʃ/? So in all likelihood the original /ʃ/ in axolotl would be pronounced by way of habit as [t͡ʃ] (unless again you have say a argentinian dialect where e.g. "ll" (/ʝ/) in llamar is pronounced as [ʃ]) if you try to "correct" mexican spanish speakers.