"Look, I've got to get back to the flat, I've got loads of studying to do."

My questions are,

1.Where we use "have got to"?
2.Can we write "I've got to get back to the flat." as "I've to get back to the flat."? If not then what is the difference in meaning?
3.Can we write "I've got loads of studying to do." as "I've got loads of study to do."? If not then what is the difference in meaning?

Please answer above questions.
Thanks