Instead of just opening them with Excel, make a blank workbook or sheet, go to the data tab, import from text/csv. It'll give you a bunch of menus and such where you can adnust how it reads the file and how it will format columns. Any column with leading zeros you want as text. It doesn't fuck with "as text" columns.

The only caveat is that once you've imported the CSV that way, you'll want to open the data sources tab and delete the connection to the csv file, otherwise changes in the original csv file will change shit in your spreadsheet, but only if excel feels like syncing it. Better to shut that off and do it yourself as needed.