Consider the following system,

$a{x}_1+x_2+x_3=-1$

$x_1+a{x}_2+x_3=1$

$a{x}_1+x_2+4x_3=b$

$($i$)$ find the values of a and $b$ for which the system has a unique solution,

$($ii$)$ and find the points $(a,b)$ such that the system has at least two solutions.