How do I alter the form submission handler?
$form['#submit'] stores an array of form submit handlers. With the code you provided you're overwriting it by adding only your own. To get the result you want you should add your submit handler as the first element in the array like this:

array_unshift($form['#submit'], 'mymodule_custom_submission');


Getting the right values in the right forms and replacing some characters is probably a bigger issue in itself. You may want to narrow down the forms/fields you need to change to make sure you don't break anything.