Thanks so much for trying this out and sharing feedback, everyone! 
To quickly crosslink with @wroos' post here, I generally do include updated translations in betas but didn't this time. We'll do another beta in early January and I'll make sure they're updated in that one.
Yes, that makes sense, thanks for sharing that. We did not include required_message in the XLSForm template by default because it's not as commonly used but maybe we should rethink that for the next template version.
Thanks for highlighting that. It's a subtle one and we didn't think about the case you described. I still think that in general it's unexpected for new form versions to have the side effect of resetting the last saved value and that this is a good change to make. Are you finding a reasonable way to work around it?
This feels a bit hacky -- but maybe no more so than the last-saved usage -- you could build in a reset to your calculation. Something like if the count of saved items is more than 100 or if the date is after Jan 10, set the value to just the new value, otherwise add the new value to the existing string.
(Related to one of my favorite XKCD comics)