Thanks for that post! I'd thought he was a military guard - but then Lincoln didn't like his military guard, did he? Then that nullifies Grant's presence as keeping him at his post. Grant had no authority over him. So, he would have been gone regardless.
No military guard. President Lincoln frequently 'slipped' his Cavalry escort and though people wouldn't picture Lincoln as an excellent horseman--Grant saw him as one and wouldn't allow anybody on his Cincinnati except for Lincoln. Grant had a huge soft spot for horses like Lee. Lee and Grant hated seeing any harm done to horses, they would go into a fit seeing anybody abuse horses. But, back to Lincoln on his horse "Old Bob,"-- He would often ride and separate himself from his escort.
On Ford Theater pictures, it is post-assassination. There were plenty of armed soldiers then. Still considered a crime scene also. Owner of Ford's Theater was coincidentally in Richmond at the time.
Unknown if Parker was assigned to guard Mrs. Lincoln. I really didn't look into that when I was on limited duty (administrative duty nursing a on-duty injury).
M. E. Wolf
By the way, didn't he actually get assigned to guard Mary Lincoln after the assassination? Wonder how that went! Did he really get that job? I certainly would object if I were her.