And that therefore, if I want to create two (or more) transactions from a watching-only wallet, such that both are spendable without conflicting with each other, then it's my responsibility to use coin control to ensure they spend from different addresses?
Yes. I do that sometimes. It works fine, if you are careful, but if you use the same input twice you cannot broadcast the second transaction (and do not get a clear error message).
We took the easy way out on this topic (and didn't address it), because the alternatives are more complex under the hood and in the UI. [...]
Yes, I figured as much. Thanks for confirming it.
EDIT: There are alternatives to locking outputs, though. You could track the transactions created and then prefer outputs that haven't ever been used in this way, and if that's not possible, prefer outputs that were used in older transactions to those used in newer transactions - and in the case where you do have to reuse outputs, issue a warning ("WARNING: This transactions conflicts with transaction xxx created on xx/xx/xxxx; you will not be able to broadcast both.")
That would mean that if you create several transactions then EITHER everything will just work OR you will be told that it's not going to work.
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