How to stage a redesigned site on a new webhost while the original site is still live? [closed]

SO FINALLY to my question: Is this indeed a brilliant solution, or is there a real possibility it will it cause me major problems like broken links and database corruption in the future?

You will find that new content created refers to the new URL, even after you remove the config statements, and old content may still refer to the original URL.

So yes, it will cause problems. You can’t change a sites URL by adding defines to the config file, the database needs migrating too with an appropriate search replace tool. ( an SQL query is not that tool ).

There are many ways to achieve your goal, this is not it.