Unfortunately my answer to the first part of your question is 'I don't know', but I do have an alternate solution.
I have never figured out how to use the new tiddler message like that, I have always used the action-setfield widget instead.
Because my brain doesn't want to focus on actual work today, I wrote a bit about how to make the button
here. It kinda rambles a bit, but it should cover what is needed.
If anyone can tell us how to use the paramObject as an input to the tm-new-tiddler message that would be awesome.