Does the output level of the reverb changes when you switch off the chorus and delay blocks, in parallel routing?
Yes it does change a bit.
Even in parallel routing it's necessary to sum the effects before the output from the DSP. Each effect adds 3 dB to the output, so if the effects are not turned down a bit when a new block is switched on you'll hit the digital ceiling (0 dB Full Scale) very early.
Of course this reduction in gain was intended to result in a steady output level at all times. However, there might be specific situations / settings where this goal is not perfectly achieved.