Help understanding the use of Linest function

You don't say how you are trying to upload the sample file. If you are trying to use the attach button, you should be made aware that it doesn't work (and hasn't for a long time). The only reliable way I am aware of for uploading files to the forum is to click on "go advance" or "reply to thread" to bring up the main post editing window. Below the message box is a "manage attachments" link that will bring up the file uploader that you can use to upload files and they will attach to your post.

I'm not sure I understand your question, or if we even have a statistician who would adequately understand the issues around collinearity.

A quick internet search found:
Wikipedia: https://en.wikipedia.org/wiki/Multicollinearity
a Penn State lesson outline: https://onlinecourses.science.psu.edu/stat501/node/345
I find a bias against Excel among many statisticians, but this guy specifically targets the Excel audience: http://www.real-statistics.com/multi.../collinearity/

There are many other internet resources for studying collinearity. Your question sounds more like you want to understand the statistics behind collinearity. I expect that once you review the statistics, programming the collinearity tests into Excel will be much easier.

As for the question of using the results, the output of a multivariate regression should be an equation something like y=m1x1+m2x2+m3x3+... Once you have the m's from the LINEST() function, you should be able to put your values for the x's in and calculate y, or whatever you need to do.

Does any of that help?