Yes, to all of your numbered points, with the addition that the guest must be departing, not arriving (the question is moot in the vast majority of cases outside the USA since lounges are generally airside in a departures area not accessible to arrivees).

Corner cases:
a) Yes for you and for a guest, but see the "Lounge Access Exceptions" on the oneworld page.
b) Case by case - it depends on the arrangement between the code-share carrier and the operating carrier. It's not covered by oneworld rules.
c) It is then judged segment by segment according to the rules on non-connecting flights.
d) No access - the key word is "join". Once you have left the lounge the guest is no longer "joining" you and must leave also.

At least, that's how I have always understood the rules over many years as a OWE (CX, BA and AA at various times) and F/J traveller.