I just learnt that alcohols can undergo $\mathrm{S_N}i$ with $\ce{SOCl2}$ (without pyridine) to achieve retention:
However, what I really do not understand is the 'internal return' step (forming 'intimate ion pair'). What drives the departure of oxygen + attack from chloride? Electronegativity of oxygen is more negative than chlorine, and bond strength of $\ce{C-O}$ is $\pu{358kJ/mol}$, $\ce{C-Cl}$ is $\pu{339kJ/mol}$. It seems to me that it is more stable to stay at $\ce{C-O}$ bond then forming $\ce{C-Cl}$ bond.
Moreover, what causes $\ce{Cl}$ to attack the carbocation on the same face? Like $\mathrm{S_N1}$, it can attack from the front and back (both sides of empty $\mathrm{p}$-orbital), what cause this to differ from $\mathrm{S_N1}$? Also, is primary/methyl alcohol unable to undergo $\mathrm{S_N}i$ due to carbocation formation?