> I have tried very diluted CP etch (HNA=1:10:9) but it always leaves a
> white-ish cloud Si surface. I don't understand why.

try to read the original article about HNA: J. Electrochem. Soc., Volume
123, Issue 12, pp. 1903-1909 (December 1976). For sure you will find how
to improve results.

> For the room temperature KOH, I leave it in there for 5 minutes but it
> doesn't seem to etch anything. I will do more trials on this. the KOH
> seems
> pretty promising.

Higly doped silicon is not etched by KOH (let me know if you need
references, now i don't have them but i can provide). Or it etched very
slowly. This could be the cause.
Another cause could be the presence of an oxide layer, try to remove it
dipping your wafer in pure HF. Just one minute will be good.
By the way, when etch starts you will see some hydrogen bubbles leaving
silicon surface.

Good luck,
Andrea