Hmm, looks like this conversation has gone in a very unfortunate direction, and rather than answering the questions, turned into a potential legal nightmare.
SuperCreek while the grapheneos position is clearly maximum security and therefore no root. As you have presented, there are some valid and legitimate uses for root. Some others include adding call recording and accessing user data for any reason (including making full backups). So I will answer your questions precisely;
1) Magisk works exactly as expected.
2) Automatic updates only will work on Pixel 7 since the changes are moved from the boot partition to init_boot, which in my experience, does not break the update validation as it does on Pixel 6 and older. However, the update will result in loss of root.
3) Take the newest grapheneos and install with magisk in the normal manner. There is nothing special to worry about.
And please pay attention to the warnings that you have been provided with and take these into account when deciding to make these types of unsupported modifications, and especially respect the fact that its not the fault of grapheneos or any of its developers if you break something in the process.