Just in case: what about my question (1)? You are plugged in to a line in and not a mic in, right?

I’m pretty sure ground loop can’t occur on a laptop when it isn’t plugged in to the wall for power. From a quick search, this seems to confirm that:

Power Supply

A low quality power supply unit can lead to ground loop noise, particularly on laptops. Disconnect your computer from your power supply so that it runs on battery power and verify if this resolves the issue. If you are going to use a replacement power supply unit, make sure that its specifications meet the ones required by your computer in order to avoid permanent damage.

I’m seeing that, beyond ground loop, noise like this is probably some other form of EMI (electromagnetic interference), often from bad capacitors. I don’t really have experience there.

I’m sure enough that the ground loop isolator won’t work that it will probably work. So try it!

If that doesn’t work, I think a USB audio interface is the next solution. Since you were still getting hum (albeit less) when plugged in to the the monitor, a new interface from the laptop should work.