The start date to calculate the commitment fee depends on the loan. Often the commitment fee starts as soon as the loan is processed and the borrower has signed on the loan.

Unused $amount x interest rate x #days in the period = commitment fee

An annual retainer fee is the cost for a commitment to have services available if they become needed. The annual retainer fee typically refers to legal services. Charges for work done is in addition to the retainer.

They can charge a commitment fee or a lock fee, most certainly. Not everyone does it but it depends on the lender, it is not uncommon though.

The reservation process in a hotel depends with your location. You can make a call, write an email or physically go to the hotel to place a reservation. A commitment fee is usually mandatory.