"If so, and the burnt matches somehow didn't get blown away in the blast, it proves that this fellow was determined to kill himself at this spot, at this time"
OR..someone was threatening him with a bomb and lighting matches to scare him and accidently or on purpose lite the final one. ??? Remember two men were seen running from the scene.
My sister's college sorority did a lot candle lighting ceremonies. My first thought on this was a group of people lighting candles and perhaps saying a prayer for the deceased.
Perhaps we're overthinking the matches thing. I haven't read anything about the condition or age of the matches (were they all new, freshly burned matches, or were they of differing ages?). It might just be that a pipe smoker frequents this bench. Or any smoker(s) who doesn't carry a lighter for whatever reason.
The matches might not be evidence at all, which may be why they weren't taken by the police.