a big part of mastering for me is listening and if your only hearing a mono mix of a stereo that seams like it might cause some unexpected results or phasing. but i am no expert in this space.
Sweet spots for eq, compression and gain will likely be vastly different between your in the box solution and the neve channel strip, your best bet is probably to get it sounding as good as possible in terms of whatever processing you want to do - eq compression etc - then run it flat through the neve and back in again - one side at a time. Set the fader to 0 and use a 1khz tone to set the input gain so your channel strip output level is the same as what you’re feeding in, then run the mix through. Should be noted as well That doing something like this will only sound as good as your converters.
i’m no expert either. Indeed this process would involve a few iterations to get it sounding right and would probably be a bit frustrating as it is impossible to adjust on the fly. About phase issues there should be none as long as all channels are sewn together back correctly and no EQ happens on the outboard strip.
Maybe i’d just squash the Mid in the Neve, and then try various proportions of “squashed Mid”, “original Mid”, and “original Side”.
Whoops I was thinking I was replying to the original comment not your response. Sorry