Hence, in FinderOuter, I have last six unknown characters and one unknown character in the middle of the key as basic search. I am replacing F with H,J,K. This search will take a very very long time for completion.
This search is trivial to do if you set up btcrecover properly. You have three unknowns for the first character, and 58 unknowns for the character in the middle. Even assuming the worst case scenario of the checksum only being 5 characters, giving you another unknown character in the 6th from last position, then there are only 3*58*58 possibilities, since the last 5 characters don't need to be brute forced since they will be calculated from the rest of the key. This is only 10,092 possibilities, which can be searched in under a second.