Imagine that you were a fruit tree.
What kind of fruit tree would you be?
1. I would be a mango tree.
2. I would like to be a mango tree.
(Which answer is correct? Are both correct?)
Do you think you would be happy as a flower? Why?
- Yes, I would be happy as a flower. Why? Cause/Because I would/will be a useful flower.
(Is this answer correct? Do I have to use 'will' or 'would' and 'Cause' or 'Because'?)
1 and 2 are both correct, yes.
Your sentences about a flower are correct, too, as long as you use "Because", not "Cause" (which is a verb or noun, not a conjuncion).
In these "what if" types of sentences, you need to use "would" not "will" because you are not writing about facts.