... As part of figuring out this I would like to measure the input of the scope to check if it is actually 15pf / 1MΩ.
...
I do not know if this is any use to you, but some measurements for you anyway so you can compare them with your results. I got the following measurements at the channel 1 BNC input of my SDS1104X-E measured with a DER EE DE-5000 LCR meter:
at 120 mV per division and above:
20.29 pF at 100kHz
21pF at 100Hz with AC coupling
22pF at 100Hz with DC coupling
0.898 MΩ impedance at 100kHz
0.9996 MΩ impedance at 100Hz
1.0007 MΩ DC resistance with AC coupling
0.9988 MΩ DC resistance with DC coupling
at 118 mV per division and below:
18.27 pF at 100kHz
20pF at 100Hz with AC coupling
25pF at 100Hz with DC coupling
0.681 MΩ impedance at 100kHz
0.9998 MΩ impedance at 100Hz
1.1978 MΩ DC resistance with AC coupling
1.0002 MΩ DC resistance with DC coupling
Unless specified above, AC or DC coupling setting did not seem to change the measurement. A check of other channels showed fairly similar results to channel 1.
Measurements with a Keysight U1272A multimeter for comparison:
at 120 mV and above:
1.0014 MΩ DC resistance with AC coupling
1.0000 MΩ DC resistance with DC coupling
at 118 mV and below:
1.1988 MΩ DC resistance with AC coupling
1.0026 MΩ DC resistance with DC coupling