I do think there would have to be some very good reasons for Tonks to lose her morphing abilities -- I really don't think it's only because Remus won't agree to a relationship

Absolutely. It doesn't quite seem to be how Tonks would react to such a refusal. And Remus has kept her at arms length for some time already - it isn't until the summer where Remus' mission begins/is settled that this happens to her. (I've read so much fanfic that I sometimes get confused as to what is canon, but do we know for a fact that she is unable to morph, or is it possible that everything that's happening around her makes her not want to do it or not have the energy and joy to do it?)
The fact that Tonks usually is so vivid and colourful - quite literally - makes her reactions all the more obvious in HBP, but looking at what she is facing - dealing with dementors in addition to knowing what Remus has to go through with - I'd be more surprised if she remained her usual self during this year.
Which makes it even worse to me when Tonks is portrayed as week. She does her job, gets out there, gets called upon by Dumbledore when Hogwarts needs protection. He'd hardly do that if she was sitting around moping.

I really loved this installment. It's so sad, and the fact that they remain so friendly, in some ways makes it even sadder, if that makes any sense.
The parallell between what Remus is asked to do and what Snape has to deal with that stereolightning pointed out, is striking indeed. Both of them are in possession of a lot of strength.


The next chapter will be better at humor, I promise. ;) There will be some Weasley antics in between scenes of only mild angst!
Sounds like the perfect combination!