sz0k30, I will try to explain as best as I can.
1. For 6mm (close to your 1/4-20) I us a 9mm drill bit. The rivnuts are not mild steel, it is some hardened metal. The 3/8 is slightly smaller than the 25/64 and therefor the interference / hard to fit. Use the correct drill bit.
2. I screw the...