Which configurations are safe to use an oscilloscope?
Your setup as you show it is not ideal but should work.  Are you really sure the laptop power supply connects its negative output with the power line ground thru 1 k&Omega;?  That resistance sounds very low.  Have you actually measured this with a ohmmeter (while everything is disconnected, of course)?  If it is around a M&Omega; instead, you can ignore this whole issue since 5 &micro;A is irrelevant to a USB power supply.

There are several obvious solutions:

Run the laptop off of its batteries while doing the tests.

Use a isolation transformer between the laptop supply and the power line.

Connect the oscilloscope ground to the USB ground, not to USB power.  It's hard to imagine why you would connect it to the 5 V power in the first place.  The difference is only 5 V, so should still leave plenty of usable common mode range from the scope, even if the signals you want to see are referenced to 5 V.  What exactly are you trying to see on the scope?