It is their report, not yours. You contract to submit it, then it becomes their property. I don't know why anyone cares what an MSC does as long as they pay. So many mystery shoppers personalize everything. .
The client is paying for an honest report.
Now, when an editor makes a change and then someone else questions the shopper about it? Yeah, that's a problem. But when an MSC makes a change because that's what they want to send to a client? Their problem, not yours.
There seems to be so much "let's be complacent, that's not our business" on this board lately. Don't worry if they change your report, don't worry about the fact that we only receive a very small share of the pie, "you" are not making enough money because "you" are doing something wrong: reach higher like "we" do. Reminds me of rotten political systems that hire filmmakers to make propaganda films.