Why do you avoid passive voice?

[Vardelm]

And a very quick google tells me that Hindi gets its ergativity from passives. (I wonder if that pattern---passives giving rise to ergativity---seems odder than it really is to some of us who've been shaped by Strunk&Whitey rhetorical traditions.)

Probably, because passives are a common (-ish???) path to ergativity.

I think the English aversion to passives (due to style guides, etc.) contributes to people having difficulty when they try to understand ergativity. Couple that with "passive voice" being defined by how the English passive works (detransitivization) makes it confusing when you have a language with a transitive, default voice that has a "passive viewpoint" (for lack of better term), where the patient is equal or higher in salience vs. the agent. I think it would be better if voice was defined more strictly in terms of salience, focus, & subjecthood rather than the morphosyntactic features it has, or there was a good term for this.

Then you describe it as ergative language with unusual antipassives.

Not quite. There's a lot more that would probably need to happen. The morphology could be viewed as vaguely ergative (with "by" or an instrumental/genitive marker/preposition being your "ergative case"), but voice markers, pronoun alignment, etc. might still show it as accusative. Syntax & semantics could still be accusative as well, where the thematic role of the subject of intransitives would need to be the patient more often than not.

I'm pretty sure that passive-heavy nominative-accusative languages still can be easily defined as nominative-accusative

Not if passive is more common than active voice.

Nope. See comments above.

It could work, I think. Start with something like this:

Sal[NOM] was-eating[ACTIVE] the-rice[ACC]
by-Sal[INST] was-eaten[PASSIVE] the-rice[NOM]

Reinterpret NOM → ABS, INST → ERG, ACC → OBL, ACTIVE → ANTIPASSIVE, PASSIVE → ACTIVE:

Sal[ABS] was-eating[ANTIPASSIVE] the-rice[OBL]
by-Sal[ERG] was-eaten[ACTIVE] the-rice[ABS]

That's a path to ergativity, yes. As above, ergative language can develop from passives of accusative languages. I think there's a line that needs to be crossed where the less-marked active voice becomes ungrammatical, and a separate antipassive voice is derived.

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Here's my 1st chance on the new ZBB forum to link my favoritest linguistic article of all time:

Typology of Ergativity by William McGregor

If you're confused by ergativity, read & study that article, and it will be much more clear.

If you THINK you understand ergativity, read & study that article, and you'll realize you didn't understand it quite as well as you do now.

If you understand ergativity, read & study that article, and enjoy.

[Nerulent]

It could work, I think. Start with something like this:

Sal[NOM] was-eating[ACTIVE] the-rice[ACC]
by-Sal[INST] was-eaten[PASSIVE] the-rice[NOM]

Reinterpret NOM → ABS, INST → ERG, ACC → OBL, ACTIVE → ANTIPASSIVE, PASSIVE → ACTIVE:

Sal[ABS] was-eating[ANTIPASSIVE] the-rice[OBL]
by-Sal[ERG] was-eaten[ACTIVE] the-rice[ABS]

That's a path to ergativity, yes. As above, ergative language can develop from passives of accusative languages. I think there's a line that needs to be crossed where the less-marked active voice becomes ungrammatical, and a separate antipassive voice is derived.

There has to be at least a change in the intransitive sentence for it to be fully ergative, otherwise you have the unmarked intransitive as:
Sal[ABS] was-sleeping[ANTIPASS]

A relevant natlang example is Māori, which is a nominative language, but with very frequent passive voice - it frequently occurs as 40-50% of verb forms in a piece of prose. It also has a lot of other traces of ergativity - for example, transitive verbs take the passive form in the imperative - and of course its close relatives Tongan and Samoan are both ergative.

Here's my 1st chance on the new ZBB forum to link my favoritest linguistic article of all time:

Typology of Ergativity by William McGregor

I keep meaning to read this and never quite making it through! I think I had it open in a tab in my browser for half a year without getting read before it got closed somehow.

[Vardelm]

I keep meaning to read this and never quite making it through! I think I had it open in a tab in my browser for half a year without getting read before it got closed somehow.

Have you read through it yet?

...

Now have you read it?

...

Nowhaveyoureadit?Nowhaveyoureadit?Nowhaveyoureadit?Nowhaveyoureadit?Nowhaveyoureadit?????

[Nerulent]

I keep meaning to read this and never quite making it through! I think I had it open in a tab in my browser for half a year without getting read before it got closed somehow.

Have you read through it yet?

...

Now have you read it?

...

Nowhaveyoureadit?Nowhaveyoureadit?Nowhaveyoureadit?Nowhaveyoureadit?Nowhaveyoureadit?????

Well.... it's open in my browser again I might even get through it by the end of the year!