What is your intention with microdosing, I'm curious.
Technically, yes I think that downregulation will happen at any dosage. But really the question is: is it
significant? I'd say almost certainly not.
Just like some chemicals have the potential to be toxic, but with toxicity the question is always how much of something you are taking and/or how often. Pretty much anything can kill you if you just take too much of it, it just varies how much you can take of something.
I wouldn't worry.
About depression: wouldn't worry either. Even if you took fully psychedelic doses weekly I don't think it's really an issue. There are more than one type of serotonin receptor and the main one that psychedelics like these act on is called 5-HT2a. If you consume way too much psychedelics then psychosis or HPPD tend to be biggest risks I guess (although some have quite good resistance even to these), and mood being affected would be much more a result of the psychological impact of tripping so much rather than a direct effect of psychedelics. FWIW, drugs like 2C-B, -C and -D are not known for causing depression anyway.
Mood probably does get affected when you unselectively affect the serotonin system, but there are also interesting effects from up- or downregulating one type of serotonin receptor while the others stay the same. I believe that is how drugs like mirtazapine work.