1.) The name of the ASC file is not important. ( When you import the key, several important fields will match the key to the right user... E-mail address, as well as KEYID are important fields that are contained in the key file. )
2.) Public key is not used for decrypting. It is used for encrypting mostly. (also used for verifying signatures)
3.) If you exported the whole keypair (Public and Private) you could use that for decrypting. Again, here the filename is not important, and all functions will work even if the ASC file is renamed.
Hope that helps. Let me know if you need any further clarification on the subject.
Best Regards,
Phil