So it looks like you haven't learnt the Vim terminology and that's the source of your confusion. Forget those "instances", "views" and such. It's all about "buffers" and "windows".
sets noma property for the file in both the editing instance end the view.
To put it properly: "noma is set for the buffer in both windows".
:h :sview: "same as :split". Now read
:h split: "If it is not loaded in any buffer, it will be read. Else the new window will use the already loaded buffer". Now to
:h 'ma': "local to buffer".
Doesn't it make the things clear? Both windows show the same buffer, hence there exists only one buffer-local option "modifiable".
This question is specific about how to set (or switch on and off) noma for a file while editing it in another instance of vim.
Again, it should be said as: "how to set noma in a specific window only"? But as we already know "modifiable" belongs to a buffer, not to a window. Hence, you can only set it independently if you have two different buffers. For example,
read ++edit $MYVIMRC
" or instead of the previous line:
"setlocal nomodified nomodifiable
Note that I wouldn't even bother with "noma", as it's a "nofile"-buffer. So even if you've modified it, it's not easy to write the contents to the disk drive. But, of course, you can also add
setlocal noma and forbid all editing operations if you wish.