I agree that the various terms are difficult to disentangle. I see the relevant taxonomy as something like this:
[S = subject, O = object, T = transitive, IN = intransitive.]
A. Not AmbitransitiveType (i): used only intransitively (a) past participle = adj:
1a. The plate fell (unaccusative)
1b. The fallen plate
1c. *The plate was fallen
1d. *He falls the plate
(b) past participle not = adj:
2a. The man died (unaccusative)
2b. *The died man
2c. *The man was died
2d. *He dies the man
Type (ii): transitive use sounds odd or humorous
3a. The colleague resigned (unergative)
3b. ?The resigned colleague
3c. ?The colleague was resigned
3d. ?He resigns the colleague
B. Ambitransitive (may be used in T and IN)
Type (i): Common ambitransitives (S in IN is agent and remains so in T)
4a. I understood (unergative)
4b. I understood your presentation
4c. Your presentation was understood
4d. *Your presentation understood
4e. ?The understood presentation
Type (ii): Alternating ambitransitives (S in the T is agent; in the IN, O becomes S-patient)
5a. He broke the plate
5b. The plate broke (anticausative, ergative, unaccusative)
5c. The plate was broken
5d. The broken plate
5e. The plate broke easily (anticausative, ergative, unaccusative)
Type (iii): Ambiguous ambitransitives ("common", but may become "alternating" with adverb)
6a. I read till midnight
6b. I read your book
6c. The book was read
6d. *The book read
6e. ?The read book
6f. The book read well ( = alternating, like 5e)
Type (iv): Pseudo-alternating ambitransitives (S in the T is agent; in the IN, O seems to become S-agent)
7a. He moved the plate
7b. ?The moved plate
7c. The plate was moved
7d. The plate moved (pseudo-ergative)
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"Middle voice" would seem to be used for ambitransitive types (ii) and (iv), and for the alternating usages of (iii).
As for "ergative", etc., I wonder whether it would help to talk about "ergative, etc. structures", rather than "ergative, etc. verbs".
MrP