a) yes, it was hot for the DI races, but it was not very hot for the DII and DIII races and the times were also slow in those races. 26:30 WON the DIII race- granted the field was very poor quality, but still. In the past that has not even been top 20.
b) The course was somewhat soft but there were no real muddy spots. Still could affect course speed though...but Griak is often kinda mushy.
c) The top guys "go for it" every single year in the DI race. The top few guys still usually run fast. I can't really recall a year in which the entire field isn't out in 5:00 for the mile.
My best guess is that the course was slightly long. maybe finish line was further back this year, or some turns were set a little too wide. Yes, the course is hilly, but remember this is the same course that Lawi Lalang ran 23:15 on. For a lot of the races, the field quality was very poor compared to previous years. This was not true for the DI women though. If you wanted to analyze times, look to that race to compare.
Can't look to HS races to compare because the 5k course is always ~100m long.