Class 1 must have their chassis connected to earth ground whereas Class 2 is designed is such a way that it does not require safety connection to earth ground.
Each LED has 3~3.5V drop. If you need to meet SELV (operating output voltage below 60V), then you are better off having at least two outputs with 8 leds in each output string which gives approximately 28V on each string. One can also run all 16 leds on one string (approx. 56V) but you would have to make sure that the output voltage never goes above 60V since you need 56V and you are cutting it very close to the SELV threshold.
HB Leds have built-in circuit whereby opening any leds would bypass the failed led so the other leds in the string are still working. You will have to use current mode control to run leds since the brightness of the led is directly related to the current flowing through it. The voltage drop across a led is non-linear.
I am not sure about the use of flyback topology for PFC. Since you already have a buck dc-dc down converter, you would be better of using a boost converter for PFC. A boost converter is easier to design. Having a flyback topology makes it more complicated but can be done. Also, using a single row, you can eliminate the use of low side buck converter for LED current making it a 2 stage converter.