Which would undergo ${\mathrm{SN}}^2$ reaction faster in the following pair and why?

Ethyl bromide and tertiary butyl bromide

The increasing order of reactivity of the following compounds towards reaction with alkyl halides directly is:

$\left(i\right)$

$\left(ii\right)$

$\left(iii\right)$

$\left(iv\right)$

The reagent required for the following two step transformation are

The major product of the following reaction is:

The mechanism of Sn1 reaction is given as:

$$\begin{gathered} \mathrm{R}-\mathrm{X}⟶{\mathrm{R}}^{\oplus }{\mathrm{X}}^{\ominus }⟶{\mathrm{R}}^{\oplus } \Vert {\mathrm{X}}^{\ominus }\stackrel{{\mathrm{Y}}^{\ominus }}{⟶}\mathrm{R}-\mathrm{Y}+{\mathrm{X}}^{\ominus } \\ \mathrm{lon} \mathrm{pair} \mathrm{Solvent} \\ \mathrm{Speparated} \mathrm{ion} \\ \mathrm{pair} \end{gathered}$$

A student writes general characteristics based on the given mechanism as :
(a) The reaction is favoured by weak nucleophiles.
(b) ${\mathrm{R}}^{\oplus }$ would be easily formed if the substituents are bulky $y$
(c) The reaction is accompanied by recemization
(d) The reaction is favoured by non-polar solvents.

Which observations are correct?

In the reaction