No, the simplest explanation is usually the best. Look at how Demonreach talks. It uses odd words for things. It would not sound right if Demonreach called it a "tree". Parasite is the sort of odd word Demonreach would use to refer to the tree.
And the distinction you refer to is hardly important. My body digests food and my parasites aid in the digestion process. Just two different ways of referring to a process at different levels.
Except that the "parasite" is specifically mentioned in connection with the circulation of blood, not of digestion. Remember the tree is associated with IV tubes going into Harry. But *something* still needs to keep his heart beating. I think that some fragment of Lash remaining behind is the most likely explanation.
Some other notes:
1. I too noticed the huge unresolved fight between Good/Evil Bob... And in the Nevernever no less, where Harry's presumably headed now. Will be interesting to see what happens.
2. Will also be interesting to see how Harry's relationship with Lea changes now that he's a fellow member of the Winter Court -- and her rival in power.
3. Am I the only one who saw this book as definitely putting the kibosh on Harry/Molly? p. 442: "...and crushes probably didn't last for years. Did they? They faded. Molly's feelings hadn't, but
I didn't reciprocate them. I loved her to pieces but I was never going to be in love with her." That seems pretty definite to me....
4. Re: did Mab lie? Uriel did not whisper "SHE lies." Just "Lies." And the Fae are more than capable of making you believe untrue things without actually lying in the form of the words themselves, as we know. So I don't think anything changed there.
5. Where the hell do the Outsiders come in to play in all of this? My current speculation is that this is all the culmination of a long-term plot by Lucifer in conjunction with the Outsiders to overthrow the established order of the Universe. He cannot hope to defeat God as is -- balance between good and evil and all that. But if he brings in a force from outside this universe, maybe he can .. tip the balance? and everything we've seen so far is leading up to that?