Grammar question

hello. I have some questions about grammar

(1) The earth's temperature has increased by about 1 degree in the past 10,000 years.

What is the difference between for the past 10,000 years and in the past 10,000 years?

(2) It has risen by 0.74 degrees in the past 100 years.

Can I change in the past 100 years with in the past century?

(3)Tuvalu, a small island nation of the South Pacific has seen two of its islands.

Can I change two of its island with two out of its island? if not, why?