Not quite sure what you mean when you say ‘amended’ but why don’t you create a ‘template’ library and create a project within this library and then copy all your titles Into that project that you want to reuse. I do this to store elements I want to reuse, or anything really that I want to have access to, without opening a large library where I initially constructed said element. If you only store titles in the library it’s super small.
Yes that’s correct. You only need Motion if you want to create title effects that then can be loaded from the title browser. Copy and paste titles between projects and/or libraries is the same result, just maybe not as convenient as via a new Motion title.
Fcpx saves every title as a self contained effect within the project. You can move or copy projects and the titles within will be there as long as the underlying Title effect is in your user-movies-motion templates-titles folder. If you based you’re modified title on a standard title effect that comes with FCPx, you do not need Motion to save a modified standard title via a library/project. Hope that makes sense.