This is a timeline I compiled some time ago. It seems like it may be useful to post it again here for reference. If you note any inaccuracies, let me know....
Your timeline seems pretty accurate as to what was reported in the newspapers of the time. A couple of comments:
Some of the times are approximate or estimates and they all come from different sources. You cannot draw any hard conclusions by comparing them.
The boy (David) who stated that he saw the girls at "7:30" walking to the mall may have either mispoken or was misquoted - or possibly he was correct in the time. As you indicate, however, he was soon discredited or disbelieved by police. They had basically written him off as early as 28 March, and so indicated in the press. To this day, however, he claims to have seen the girls, but I do not know what time the feels he saw them.
The two boys who stated that they saw the girls walking down Drumm near Devon between 2:30 and 3 PM actually came forward within a day or two after 28 March, but police did not release that sighting information to the press and public for about 2 weeks. Thus the 2 week delay mentioned. One of those boys was described as "Over 15" and a school friend or acquaintence of Sheila. The other boy was described only as the driver of the car, who corroborated the incident of seeing two girls walking down Drumm, but he did not know either girl before their disappearance. For a more detailed post on this see:
The "Over Fifteen" story: Last reported sighting on 25 March 1975...
in this thread: Potential Suspects and Persons of Interest - Page 4 - Post #98
LINK:
http://www.websleuths.com/forums/showthread.php?t=35536&page=4
Regarding the statement of James Mann, I have not found any reference to his sighting them in any source prior to the 2005 30th anniversary article. I have confirmed that a James Mann did live at that address (corner of Drumm and Deven) both in 1975 and in 2005.
One statement (which came from the Washington Post) about "she was sitting on the Easter Bunny's arm as children talked to the bunny" has always struck me as odd. I think what was meant was that she was sitting on the ARM of the Easter Bunny's CHAIR, listening to children who would have been on his lap or next to him on the chair.