Yes, that's how I traced it out, and was concerned for two reasons, the possible mixing of "noisy" and "clean" grounds, and having too many things on that ground post, which might make the connection less secure, which, with the safety ground in particular, is bad news. A loose signal ground would cause noise.
I think you mixed up things here. As I already wrote the signal ground is completely on board (as the connection to "noisy" power supply), so it can't be loose and noisy.
According the rules for safety ground it should be connected near the mains earth input either by it's own nondetachable connection or with other connections, but the wire should be nearest to the metal part of case and be connected by different means than other wires (that means you should be able to disconnect other wires while the earth wire stays safely connected).
I'm agree that the electrical contact with th case should be good, that means you should remove paints, anodising layers and other obstacles which prevents the good connection.
Note the hole pattern for the RCA input and output modules (if you were using that part)... There are 5 holes per module, so one each for:
- left shield
- left signal
- ground post
- right signal
- right shield
No, if you look through the datasheet for the part you find, that leftmost and rightmost hole is for mechanical purpose only. The central pin is common ground, left and right - corresponds to respectable signals.
I think you should begin with changing wiring for inputs and outputs, then measurements, then anything else.
