Dave, go right ahead and use the material you require for your file.
As for the grave site, I do believe that, if it is on common ground, that a marker cannot be placed at the actual site of the grave. I did try this with a Civil War veteran's grave in South Australia, many years ago, but because of certain legal technicalities, was not permitted to place a marker at the actual site of the grave.
No, I have not read LAST FLAG DOWN, but if it quotes mostly from Whittle's diary, I already have that item in the volume THE VOYAGE OF THE CSS SHENANDOAH: A MEMORABLE CRUISE, so it would be pointless in me obtaining LAST FLAG DOWN unless it has much completely new material, not shown in other sources. But I suspect, as with most books, it would only be repeating what I already have in many other sources, some of which are first hand accounts. I much rather prefer the first hand accounts, and especially copies of the original handwritten diaries, journals or letters, as they tend to be much more accurate, and, additionally, I can often try and decipher some of the words which others may have incorrectly interpreted. I was able to do this most recently in a word in the journal of another officer of the CSS SHENANDOAH, John Thomson Mason, which the Museum of the Confederacy has online. The MOTC transcription originally indicated that they were unsure of the name of a place in one entry, so I purchased a copy of the original page from them, and was able to immediately decipher the name of the place as being Sidney [Sydney] in New South Wales. Since then they have corrected this word. I also have the entire xeroxed copy of the Charles Lining journal, which the MOTC have given me permission to transcribe and place online, but I am still having quite a bit of trouble deciphering some of the words, and I also want to include extensive notes on persons and places named in the journal, so this will take a long while to complete.
By the way, if you are indeed also studying vessels of the Union Navy, you may also want to correspond with a gentleman in North Carolina, who is doing much research on the vessels involved in the sea campaigns there in 1861 and 1862, and has been trying to put names to some of the vessels shown in some of the US Navy photos available at the Naval Historical Center web site. Bruce Long is quite thorough in his work, and he has also uncovered some previously unseen photos from some of his private sources.