I gather it won't let you do a clean uninstall (since you said it's not showing any other payment modules, likely due to the PHP error),
so I wonder if doing a complete removal directly in the database might help. You can try the SQL Patch tool with:
This is to remove all non-AIM configuration settings:
Code:DELETE FROM configuration WHERE configuration_key REGEXP '^MODULE_PAYMENT_AUTHORIZENET_[^(AIM_)]';
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