My views:
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we should definitely do 1. given its prevalence, even though the citation format is stupid. There are still parts of this I don’t fully understand (e.g. what happens when you have a group of references and one of them has been cited previously? 1b, 2?), but I hope we can figure those out. Thankfully I think since this is basically all chemistry, all such styles do follow the same logic so we don’t need to accommodate variation within this.
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This is a messy one. So the rule would be "if >1 item of type chapter or paper-conference from the same container-title is cited, use container-title-short and include the full book in the bibliography. Seems tricky in terms of both syntax and implementation in citeprocs, but if we find reasonable syntax and citeproc authors don’t balk, I’m OK with supporting it. I wouldn’t do it, though, if it turns out to be highly involved, since it’s fairly rare and a bad idea (bibliography entries should stand by themselves if you think e.g. of parsing them automatically)
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Imo this is actually the most legitimate of use cases in terms of the citation style logic, but I continue to think this is more appropriately handled by the reference managers, entirely outside CSL (though might be that citeprocs need a hook to help with it). The example of how BibTeX does this given in that thread is a good illustration why: often the headings follow custom keywords or collections/folders that will have a different logic for different tools. Users should specify the groupings and headings in their reference manager and then assign items to them using a suitable logic.