I'm pleased I did this one: my first DLM. I found it *really* slow to start: I don't really like definition-only, and the fact that the third section of each clue only *contained* the jumbles thinned the clue help away a bit. I needed a few hints from here (thanks!) to help me locate the first few definitions, but I was intrigued how once I started to slot them into the grid the pace really picked up. I was lucky, and stumbled across the name of the genus, hence Latin name, of the second titular plant and so spotted the line that needed a letter changed, and that gave the idea away so I guessed what word might be defined: my second guess was right! It's right that part of the highlight is a looped item: it is, after all, of them all, the one that does what it does!
There are a few things I thought were awkward, which spoil an otherwise well-crafted puzzle for me:
1) there are two possible routes for the definition at the SW corner, and you have to use the fact it's a "decagon" to choose which to use: a bit persnickety that.
2) there are two occurrences of one of the nine items, and you have to use the fact that they are in "nine distinct rows and columns" to choose which to use -- which you can only do once you find all the others! A bit mean that.
3) as murky pointed out, the preamble is not quite accurately phrased: the highlight is a cryptic representation only of the SECOND of the two key lines of the carol, while the decagon and the nine looped items embody the first of the two lines and provide the context -- that is so cleverly done, and such a pleasing device, that it's a shame that this minor editorial slip slightly detracts from it.
But, hey, I got there -- and I'm glad to see we're given a few extra days so there's a chance I can still post it off! Only now can I dare open today's....!