What you have us an updated version of the issue outlined in this thread.
Your trouble is IOS60 stub rather than IOS50 stub. Updating to 4.2 updates your IOS60 to v6400 (a zero stub file), downgrading to 3.2 makes no changes to this file. When you update using NSMB it checks it's IOS60 v6174, sees it as lower and ignores IOS60 as an updateable IOS.
The consequence, a full brick only rescueable with bootmii/boot2.
Check your Wii for dual DVD slot flashes (very fast flash). Bootmii requires a compatible SD card, if yours doesn't work and you have boot2 installed then change the SD card.