I read that Kansai Thru Pass is only available at certain times of the year.No, it is available around the year!
Also, can I get the pass in advance prior to usage? Eg. If I plan only to use it on 14th Oct, can I purchase it (in Japan) on 11th Oct and activate its usage only on 14th or is it immediate activation on the date of purchase?
You do not need to activate it immediately.