In my experience the answers will vary based on the specifics of the airlines, connecting airports and the knowledge / willingness to work of the agents checking a pax in.

I have been able to interline bags between AA and SQ in NRT and AA and LX in LHR, neither of these are OW and to make matters a bit more complex, these were separate tickets, so the logical answer to scenario 1 is Yes.

The answer to scenario 2 should be Yes / No. Yes, if the agent checking you in does not realize that this is a codeshare with ZZ (no interline agreement exists with AA) and thinks the bags will be routed to YY. I guess it will be up to YY staff then to move the bags to ZZ. No, if the agent knows about the missing interline agreement and refuses to check bags through.

Finally, sometimes baggage tags will need to be written by hand because the systems does not like the routing, halts, etc. Some agents state that they cannot check bags through because the system will not let them and you could be stuck with collecting and re-checking your bags.