sorry for the late response and thanks for reminding me to answer this. Very good questions indeed.
(1) No, at the moment there is no hint for the second user that some string has already been submitted by another contributor. And to be honest, I am afraid that adding such feature might have negative impacts on the AMOS translator filter performance. What generally helps here is that the maintainer deals with submitted contributions promptly (so the approval queue is short). Note that while reviewing the second contributor's strings, they will be displayed as modifications of the existing translation (which was probably does by the first contributor) so the maintainer can choose the "better" of those.
(2) To be 100% sure about the answer, I would need to write a unit test for this. But I tend to think that AMOS keeps the original timestamp of when the contributed string was translated. And it uses these timestamps to display outdated strings then (if you look at the string timeline, outdated strings have more recent English version in the left column). So yes, I expect that the string would be marked as out-dated.
(3) At the moment, AMOS does not support such notifications. I was thinking about a similar forum as we have for the notifications about core strings changes coming from moodle.git. But there might be a workaround for you soon. Aparup, our Plugins directory guru, plans to implement new RSS feeds generated for a particular plugin. You might watch such feed to be informed about new versions available. The other way (which would work now already) is to have the plugin installed at some your Moodle site that informs you about available updates. The point is that new strings in AMOS appears only if a new add-on version is published in the Plugins directory. So once a new version is available, you can go and check it's all up to date. I can also imagine some "watch this plugin's strings" feature directly in AMOS but I can't promise I would be able to work on it anytime soon.
Hope this at least clarifies some parts. Please do not hesitate to elaborate your questions should some things remain unclear.
Thanks for all the strings!