Having a master project open locks all subprojects, even though we did not 'link to project'

Can I suggest a slightly different approach? When creating your snap shot I would copy all the plans from SharePoint into a local drive or maybe something on One Drive. As you're only interested in looking at the combined master it doesn't matter that you've taken copies. In this way nothing that you do can possibly impact anyone else and they can continue to use their sub plans as normal.
When I am acting as a human MS Project Server/on line I will have three folders: Input, Programme plan and Output. I receive the version controlled plans from the PMs into the input folder, open then and save as into the Programme Plan folder over writing the standing copy in there which doesn't have any version control. The master programme plan is a master plan with the plans inserted and linked. I do what ever I need to do on the Programme plan including making changes as needed to the sub plan, running reports etc. When I am done I save each sub plan out into the "output" folder with a version control and return it to the PM.
In this way I can also have a resource pool running in the Programme Plan (though I have to remember to remove the association of the output files to the resource pool file and also be careful that the input files don't contain "new" resources which are actually duplicates of existing one).  I did a bit of a blog post explaining this method: https://www.summarypro.co.uk/blog/working-with-a-resource-pool-and-distributed-project-files.aspx
This 3 folder method is useful but would probably be overkill for what you need as you're only interested in a snap shot and aren't interested in editing the sub plans etc.