Hi Tom,
After reading your questions, I was going to recommend you post them at the Canon Talk Forum at dpreview, but I see you've already done so.

That's great because your questions are more suited to that type of forum.
The Q&A Board is geared to primarily beginner and intermediate digital camera owners. We deal with more general and "how to use" questions than ones dealing with the pluses and negatives of a particular brand of digital camera. When we get those types of questions, we usually refer them to places like dpreview or Steve's digicams which are geared to handle them. I do hope you understand.
I do happen to have a 5400 and use it a lot for macros. Think it does an outstanding job. You can see some of my macros here but keep in mind that I am not a pro.
http://www.pbase.com/gailb/scrub_floraLike many other digicams, the 5400 does have difficultly focusing in low light. It is more often due to low contrast than low light. One should determine how often they take photos in low light so see if it's as big of an issue as some make it out to be. For me, it is not. And I use the work arounds listed in my Nikon 5400 BLOG at the main site.
I am sure you will obtain the answers you need regarding your brand-specific questions about adapters.
Sorry we can't be of more help with some of your questions.