The four memorials are Blue Beetle (Ted Kord), Aquagirl (Tula), Robin (Jason Todd), and Artemis
Lex Luthors head of security Otis is a reference/homage to Ned Beatty's character, Otis, in Richard Donners Superman (1978).
Donna Troy was supposed to make an appearance at Raquel's shower but creators didn't have enough time to create her look and story.
The events depicted occur on March 21st.