Original configuration
Signal to harmonics: ~10 dB.
Signal to "floor": ~20 dB.
 
Change C7/C8 divider from 33/220 pF to 150/1000 pF
Scope shows signal at XTAL1. (DC offset was due to scope mis-calibration.)
Signal to harmonics: ~10 dB. (Lower density of harmonics.)
Signal to "floor": ~20 dB.
 
Added resistive probes to measure clock phase
(5 kΩ on "yellow" CLK, 500 Ω on "blue" CLKM)
Signal to harmonics: ~15 dB.
Signal to "floor": ~23 dB.
 
Disabled CLKM
Density of harmonics decreases further.
 
Added 10 kΩ in parallel to C8
Scope shows averaged signal at XTAL1.
Note: changed FFT ref level.
 
Added 0.5 pF bypass to antenna feed (at "knee")
No perceptible change.
 
Change C7/C8 divider from 150/1000 pF to 220/1000 pF
Scope shows average signal at XTAL1.
No significant change.
 

Commands used in all cases:

atrf-txrx -f 2450 -p 3 -T -0.5  
atrf-txrx -f 2450