By how many pKa units will the pKa value of a ketone be minimized by addition of a Lewis acid
I used to work in a group that specialised on the total synthesis of polyketides. For obvious reasons, syntheses of polyketides rely on aldol additions a lot. Yet, I never heard of any simple rule of thumb for the $\mathrm{p}K_\mathrm{a}$ value differences so I suspect there isn’t one.

Both boron reagents enolise ketones practically by themselves, $\ce{NEt3}$ is typically added to capture any excess protons. On the other hand, I have never heard of lithium or sodium enolising a ketone by themselves.

Finally, $\ce{HCl}$ (or any strong proton source for that matter) will protonate the ketone but not to the enolate. Instead, the protonated ketone with $\ce{C=\overset{+}{O}-H}$ will be obtained.