1) Difference between LT3755IMSE-1#WPBF and LT3755IMSE-2#WPBF
On page 1 of the datasheet you'll find this statement:
A sufficient answer to question 2 does need some more details. LED string info?
From these two values it can work from VIN range and current value, but again: Give more details on the complete application.
Depending on your LED string+voltage a smaller integrated part can do the job as well (and easier and smaller)