There would have been no assualt had Lincoln not sent that War Fleet. It was that war fleet with orders to attack that started the war. Anderson himself said as much.
I have also recently read something that said Anderson was about to surrender. It said he penned a surrender statement. I think I still have it on one of my open tabs somewhere. If that war fleet had stayed away for a few more days, Anderson would have surrendered the Fort without a shot having been fired.
Don't knock yourself out looking for any "penned surrender statement" by Major Anderson. I'm sure it was only a daydream you had. Major Anderson had sent Beauregard a letter saying he would evacuate on April 15th if he didn't hear anything countermanding that from his Government. I am certain that is what you are referring to. The Government in Montgomery didn't go for that idea. Please pay particular attention to the use of the words "evacuate" and "surrender". There is no way in a million years that Major Anderson would have surrendered without a fight, short of starving to death. But I'll look forward to you producing the "penned surrender statement".