This is really cool! First I think this is really valuable, and the transcluded content should be visually distinct to make it clear it’s from a foreign source.
Some answers to the questions you pose in your document:
Should transcluded content be selectable, having the same behaviour as the rest of the content? Yes it should be selectable as normal text, for copy paste like behaviors.
Should transclusions be clickable, acting like an embedded tweet? There should be a little icon or UX affordance that when clicked on takes you to the source material. The entire block of content shouldn’t be clickable, as that can be annoying and can get in the way of things like copy/select or touch scrolling behaviors on mobile devices.
If clickable, what should clicking a transclusion do? It should simply take you to the source content, replacing the current view. I’m not a big fan of side by side comparisons ala Xanadu, in practice personally I haven’t found that UI affordance that useful.
Keep the UI simple, elegant, and useful.