I haven't dealt with an airbags deployed car so my experiences might be different, but somewhat relevant.. I can clear codes with my cheap autel hd200/ap200 on a car showing hv system workshop dash message, and shift into neutral, but the autel won't clear codes that are still a persisting issue. C3 STAR same thing, won't clear more critical codes that are still an issue. To shift to neutral you might also try clearing codes with any generic to mid range scanner. Or try disconnecting the 12v battery, wait a few minutes, reconnecting, and trying to shift immediately as soon as you turn on key - from what I recall that's all I had to do (foot needs to be on brake to shift as well I believe) on a car with hv workshop message. The autel does see all systems and have some advanced options and live data though.
Most if not nearly all airbags deployed cars in auctions and junkyards seem to have a dead hv battery, (typically sitting 3+ months), it's not clear what the timeline is like for fully discharging but the clock is probably ticking in your case. I would be getting the 12v on a trickle charger asap or periodic charging the 12v battery with another car (jumper cables-ED key off) as a minimum..
I'm not sure what impact the (under dash) hv disconnect has but don't turn on keys when it's disconnected, as that might cause additional error codes.
I believe there is a feature using MB STAR/Xentry where you can turn off/disconnect the 12v battery and/or tell the hv battery to go into long term storage, but I can't currently find the info on this. Again it's not clear exactly how this would play with your airbags deployed scenario, it's possible it could still discharge the pack.
If you can verify the pack is still holding a charge, an option to consider might be disconnecting all the battery connectors under the car, not too hard to do once it's on stands. I would think (but don't know) this might prevent the pack from discharging.
I would appreciate anyone sharing other experiences/comments, it would be nice to get a better idea on "proper approach" to preserve the hv pack, and a future resource for anyone in the same boat. I believe there were at least 1 or 2 other people posting online about working on airbags deployed cars at some point but I can't recall any that came back to share a solution/final details etc. If there is any relevant info out there I'd be interested in seeing it.