Maybe you have already found a way to help you with the case insensitivity issue in excel working with the Salesforce 15-digit id's or maybe you haven't?
I was looking for a solution which helped me do VLOOKUP in excel to add data from other objects, based on the 15-digit id and I thought I'd share it with you.
http://www.vandeveldejan.be/how-to-deal-with-the-case-insensitive-vlookup-in-excel-when-preparing-a-salesforce-datafix/Feel free to share if it helped you ;-)
@Admin Group, Brussels, BE