I note the options are not entirely exclusive. For example, Option 1 and Option 2 could both easily be done.
My opinions:
- Option 1: Yes, if we're doing this at all it should be available in Scribunto. Unless we make a new templatelinks-like table with all the infrastructure that entails, templatelinks is the way to go.
- 'pagelinks' is used as "the linking page needs updating if the linked page is created or deleted", while 'templatelinks' is used as "the linking page needs updating if the linked page is updated". This sometimes conflicts with UI-level requests for certain behavior at Special:WhatLinksHere, such as "only links not due to templates" or "only directly-transcluded templates".
- Option 2: Maybe. This would likely need the same templatelinks-like functionality, BTW.
- Option 3: Probably not necessary with 1 and/or 2.
- Option 4: There is a call for this access.
- Option 5: I agree with the self-characterization in this option's first sentence.
- Option 6: This has nothing to do with accessing the properties.
As for the concern over indeterminate behavior, I'd recommend (1) accessing properties of the page being parsed should either return an error or return empty/unset/default, and (2) don't worry about it beyond that since it's probably not worth the trouble.
We should also be careful that the update of the "transcluding" page happens after the page_props table is actually updated (I hope that's already the case anyway, but in case it isn't we'll likely need to fix it somehow).