I have not tried using an excel spreadsheet, or any document, with lockdown browser, and so I recommend testing this as a fake student (not just student view) before relying on it with a live class.
To modify the instructions for canvas, you should be able to save the file in the files section, and set the permission to "only available to students with a link." Then insert a link to the file into the question with the built in canvas tools. In the new rce the file insert functions look like this screenshot:

If you do not see these options you can turn them on in settings > feature options > RCE enhancements.
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