I am a piano teacher and do consultancy work for the ABRSM. I have certainly never heard of any rule that you have to pass every section to pass overall, so I really don't think there's anything in that.
These days teachers can log in online to get results (but only the marks for each section, not the comments) before they are posted out. If your daughter's teacher is not aware of this, perhaps you should tell him/her.
As it happens, my son also did his grade 3 around the same time. As I hadn't heard after three weeks, which was unusual, I logged in and there was a big notice saying sending results out had been delayed because of the snow! I think they were all over the place because many kids had not been able to get to exams and they'd had to rearrange a lot.
Anyway, as it happens he also got a merit!
Sight reading is almost always a massive pain in the butt, for PIANO SPECIFICALLY. This is because you have to read two lines of music at once, which involves a whole layer of cognitive difficulty beyond reading a single line. Most students in the early grades struggle terribly with it. My personal opinion is that many take grade 1 much too early, and they need far, far more experience reading single line music before trying to read both hands at once. Reasonably talented kids learning single line instruments often hardly have to put any work into it, but on the piano even they invariably do.
Your daughter will probably find sight reading on the clarinet incredibly easy by comparison. She will still have to address the difficulties on the piano though, and if she doesn't do so soon she risks never being able to, because her ear will just take over everything.
My son got 18/21 for sight reading, but only because after failing it at grade 2, I sat him down and did virtually nothing else for six months!
Which some might call child abuse...