I did know about the web site which you have mentioned but since it’s online, I am not comfortable checking out the checksum of the private key which I have in order to gather info of the corresponding pubkey address.
You won't be able to calculate a valid checksum for your private key since it contains invalid characters. That website will simply throw an "Invalid Base58 encoding" error if you try.
I am currently using FinderOuter offline but I have not come across checksum of pub address or private key, since the private key I have has been showing checksum errors on several occasions.
As I covered above, this does not mean your checksum is incorrect. A checksum error simply means the checksum does not match the rest of the key. Given that the rest of the key is invalid as it contains invalid characters, there is no checksum in existence which will be valid. Every possible checksum will return the same error. You are focusing on the wrong things here.
As I mentioned earlier on my posts, my key starts with 5 but is followed by ‘F’ and has ‘I’ included as one of the characters and ‘l’ on one of the last seven characters (which I presume as checksum) of the key.Please correct me if I am wrong.
The checksum encodes 8 hex characters in to base58 characters, meaning it will be either the last 5 or 6 characters which are the checksum.
My current position is that I am unable to share this key with any recovery/investigation due to unknown public address key and their total number of bitcoins.
I am happy to attempt to brute force it for free, at least for all the straightforward character replacements and similar we discussed earlier. Completely understand if you don't want to risk it though.