What is going on in this VCXO circuit?
Invariably, using a cascode like this is to overcome the effects of the miller capacitor in the low transistor - it's collector is held at a fixed bias point by the emitter of the top transistor and, the top transistor's base is fed DC. The fact that it couples via R955 to the crystal is unimportant - it's important that the base of the top transistor is higher than the base of the bottom transistor or it won't be biased correctly.

C952 is in parallel with the varactor and it basically adds to the varactor capacitance thus making the range of the varactor a bit less than if C952 wasn't there.

C958 is there to make that whole varactor setup appear strongly capacitive - without C958 shunting R960 it probably wouldn't oscillate. Why R960 is there at all is the most puzzling but I expect it is to stop the AC waveform superimposed on the DC control becoming too distorted (as it passes through or close to zero volts) because of the potential rectifying effects of the varactor.