Just before shadow instance creation SUM prompted for 'lock the development phase'. So can our consultant's work ?
After this phase you will not be able to bring in any modifications (transport requests) to this system.
What I see is that you are nearing the "Downtime Phase". The upgrade tool will stop the shadow instance before the down time transfer. This is normal and you do not need to start it if you see it not running. Also the main system is still available and only with the above restriction.
There is no need to be under pressure as this is how the upgrade works and you just need to stay calm.
Regards
RB