Nabana no Sato from Osaka

Hi
Im planning to visit Nabana No Sato from Osaka on a day trip on either 31 Dec or January
My questions are :
1. Which dates is better in terms of crowd and park hour?
2. How do i get to Nabana No Sato (prefer to take train as much as possible)
3. If i use shinkansen will i be able to use Kansai Pass for the train ride?

Many Thanks in advance

1. Which dates is better in terms of crowd and park hour?

Crowds are larger on Friday and Saturday evenings as well as evenings before national holidays.

2. How do i get to Nabana No Sato (prefer to take train as much as possible)

By far the most cost-effective approach is by Kintetsu Railway, departing from Osaka-Namba (also stopping at Tsuruhashi) and getting off at Kuwana Station:
http://www.hyperdia.com/

It is not covered by JR passes or the Kansai Thru Pass.

3. If i use shinkansen will i be able to use Kansai Pass for the train ride?

Not sure what pass you are referring to, but no, no pass with "Kansai" in its name will cover the shinkansen ride (or any other train ride) to Nabana no Sato which is located outside of the Kansai region.