Derivation of Maid, Maidli, Meydli

Unless I'm really missing something, German Mädchen is not a descendant of Proto-West Germanic *magadīn. Rather, German Mädchen is from German Magd + diminutive suffix -chen. German Magd is descended from Proto-West Germanic *magaþ.

Also, English maid is not derived from maiden, but is instead descended from *magaþ, same as German Magd. Meanwhile, English maiden is descended from *magadīn. The etymology currently given in the maid entry appears to be incorrect.

Well, it seems that this overall topic is somewhat more complex than I thought. So according to the answer from Eiríkr Útlendi, the following Mädchen entry in magaþ is also incorrect.

• Old Dutch: magith
  • Middle Dutch: māget, meit
    • Dutch: maagd, meid
• Old High German: magad
  • Middle High German: maget, magt, mait
    • German: Magd, Maid
    • Luxembourgish: Mod
    • Yiddish: מויד (moyd), מאַד (mad)

It would be really great if someone with a linguistic background can check & correct the whole matter.

However, at least the following derivation should be correct:

Alemannic Maydli is from (old?) German Mayd + diminutive suffix -li. So, the Alemannic term Maydli is effectively descended from Proto-West Germanic *magaþ.

Derived, not descended. Descent implies that the same word continued to exist and be propagated through the generations.