What makes the spelling of "psychedelic" a mongrel spelling?

According to this Wikipedia article, Richard Evans Schultes thought that psychedelic was a mongrel spelling of the word. The other option was phanerothyme. Apparently, they are both constructed from Greek words for "spirit" and "manifest". In the case of psychedelic, psyche- is Greek for spirit, -del- is derived from dēlos, which is Greek for manifest. In the case of phanerothyme, phanero- is Greek for manifest, and -thyme is Greek for spirit.

How come there are two spellings derived from Greek, and why was one of them considered a mongrel spelling by Schultes?

The sentence in the Wiki article is:

This mongrel spelling of the word 'psychedelic' was loathed by American ethnobotanist Richard Evans Schultes, but championed by Timothy Leary, who thought it sounded better.

The reference Wiki cites for this statement explains that Schultes was objecting to the spelling, not the Greek derivation:

Schultes cautioned Leary that the word, meaning "mind manifester," was appropriate, but the spelling was incorrect. The proper Greek was "psychodelic," and Schultes was concerned lest a Harvard man be associated with the bastardization of a classical language. Leary suggested that "psychedelic" sounded better. Wade Davis; One River:One River: Explorations and Discoveries in the Amazon Rain Forest p.120 (1996)

The OED gives the etymology as:

Irregularly < ancient Greek ψυχή psyche n. + δηλοῦν to make manifest, reveal ( < δῆλος manifest, visible: see delomorphic adj.) + -ic suffix.

M-W and Lexico also label the formation as irregular.

Note that the other (non-adopted) proposal

"phanerothyme," from the Greek terms for "manifest" (φανερός) and "spirit" (θύμος)

begins instead with a combining form, "phanero-", as "psychodelic" would.

If the Wiki contributor had written, say, unorthodox spelling instead of mongrel spelling, perhaps readers wouldn't be confused.

It's not just the e that's a problem. -delic isn't right either. It should be psychodelotic.

See Nick Nicholas's erudite and amusing rant on Quora.

Summary:

1. Greek uses o as a linking vowel.
2. delic is derived from the verb stem del-; the appropriate combining form would be -delot