1. This is sorely to reduce error as larger amplitude will increase angle of deflection thus creating much space for blunders. 
2. Conditions;
* The object must exhibit periodic motion that is to and fro motion about it's mean position. 
* The restoring force must be directly proportional to the extension and must act opposite the direction of the motion. 
* The acceleration must be directly proportional to the extension and act opposite the direction about the mean position. 
3. It's known, 
[tex]t  = 2\pi \sqrt{ \frac{m}{k} } [/tex]
from 1st eqn, when mass is doubled then m will be 2m
and, 
[tex] {t }^{2}  = 4 {\pi}^{2}  ( \frac{m}{k} )[/tex]
thus, 2m will increase T by factor of √2.