@Ross at Play
1. 'who' when referencing any person(s) or anything people-ish, e.g. vessels, animals, aliens or demons, institutions making human-like decisions
For me, that depends on how 'human' the aliens are. If I refer to an alien character with standard pronouns (i.e. either "he" or "she") then I'll use "who". But if I use the pronoun "it" (because they don't have our binary sex organs), then I'll use "which" instead. (All you non-scifi nerds can ignore that difference, though).
2. then 'that' when pointing at a specific subset, i.e. introducing "restrictive" information; If in doubt then conclude it does not
In that case (the "that" refers to something stated previously), then you should also consider whether "it" is more appropriate, as it often is, but most people, in their hurry to finish up, never even consider the difference. Again, if you referenced a 'thing', a concept or a declaration, the "it" is a decent replacement.
2. then 'that' when pointing at a specific subset, i.e. introducing "restrictive" information; If in doubt then conclude it does not
Those are two completely different uses of "which". For anything requiring commas, it is a restrictive clause (if A, then B, or when A, then C), but if it doesn't, then "which" merely refers to non-human 'others' (like animals, certain aliens or concepts or ideas).
Any comments on that idea?
Only that, for fiction, I'd also break it down into comma-delimited and em-dash delimited, but that's a separate (and overly complicated) discussion.
When Is 'That' Safe to Delete?
This is surprisingly simple ...
When followed by a subject then a verb!! That is basically it!
I hate to cause trouble, but if that that refers to something reference recently (same or previous sentence) then "that" becomes the noun-phrase, as it's short for 'the topic we were just discussion' or even 'the person {or alley-cat) we were gossiping about'. In that case (ex: "That's the point, we should eat an Antonio's."), the "that" is necessary, since the sentence is meaningless without the principal noun-phrase.
But it's yet another complication to consider (so I don't blame anyone who doesn't want to bother evaluating between them).
Some of you guys don't pay me anywhere near enough for some of the work I do here, in neither money nor respect. :-)
Trust me, despite the fact I constantly give you grief, I do appreciate the extra research you do (but mainly, because you'll the only one who will put up with my tirades on these sorts of issues!). 'D