Which is the most reactive halide towards $S_{N^1}$ reaction?

The structure of product $(A)$ is:

Which is the correct decreasing order of boiling points for the following compounds?

The $S_N2$ product of the following reaction is:

$KI$ in acetone undergoes $S_N2$ reaction with each $P, Q, R$ and $S$. The rates of the reaction vary as:
$P: CH_3-Cl$
$Q: (CH_3)_2CH-Cl$
$R: CH_2=CH-CH_2-Cl$
$S: C_6H_5-CO-CH_2-Cl$

Identify the major product of the dehydrohalogenation of $CH_3-CH_2-CH(Br)-CH_3$ using an alcoholic base.