usually params.ini unwritable is a common issue in joomla, you should be able to change permissions for that file through your ftp program to make it writable again.
in the newer templates you can disable the logo in the template parameters and instead use the area where the logo was as a normal module position. that would allow you to create 2 different costum html modules, each containing a different version of your logo, which you can then assign to be displayed on different pages in your site.