Most DAWs of todays do the processing in higher bit-depth accuracy. Although you should consult each DAW's manual or technical support for this, I believe it is reasonable to consider that the processing happens with at least 32-bit depth.
Even more, the internal representation of the samples is something that also needs to be taken into account. 24-bit PCM data is most often represented in integral (integer) values. The conversion from integral to floating point numbers is not necessarily trivial (and most often than not is not trivial). Even if the DAW developer/manufacturer uses IEEE's standard (even without subnormal numbers taken into account) the representable numbers is way more with floating points than with integers (of 24-bit depth).
Thus, you will most probably not get any noticable degradation of the audio material from attenuation of 20 dB.
Regarding the additional processing you mention, this largely depends on the nature of the applied processing. Most probably, you won't get any degradation in this case either, but this has also to do with the implementation of the processor (VST, TMD, AU, etc. plus the way it was implemented by the developers).