Alcohol protection with ethers
You are right.In case of benzene and PMB, the delocalization is not good enough. So a strong acid is required.

As for the case of OMe, a strong acid as well as strong nucleophile will be needed. A weaker acid can't do the job. Let me explain why.
Note that carbon oxygen bond is quite strong. 
So what we need is a strong nucleophile. But in case of OMe as a protecting group, even a stronger nucleophile will not work if the oxygen is not heavily protonated due to two reasons:-
1. Strength of C-O bond.
2. SN1 can't occur as methyl carbocation not stable.

Here acid is a stoichiometric reagent, not simply a catalyst. If a weaker acid is used, it will just establish a protonation-deprotonation equilibrium. However, a stronger acid will shift the equilibrium towards protonation.
And haloacids are best suited.

Conclusion being that a weaker acid will render nucleophile incapable to cleave ether in case of OMe.