"to whom it may concern", can i write this as well as the name and adress of the person I am writing to?
the letter is to a solicitor but I would like to indicate that he can use it to show to other parties...
No, don't do that.
You can put in the body of the letter that he has your permission to show it to others, but the letter itself is written to your lawyer.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.