Today I was able to successfully reinstall Piwik. I am not sure how much of the following will be host dependent but hope it can help you.
After talking with support I I made the recommended changes using the code editor in CPanel->File Manger. (Note that before I made the changes via FTP & was unable to get the changes to apply.) After waiting for a few minutes, I was then able to complete the Piwik installation.
I suspect that using the code editor triggered a refresh of the php.ini file by the server that editing it via FTP did not.
Everything is now working as before.