For me, I would want a sponsor to tell me in writing that they want to elect the alternative method. Then I'll prepare the premium filing using the alternative funding target. There is a checkbox to say that you are electing the alternative method, which would need to be checked.
The rates for the alternative method are just the 430(h)(2)(C) segment rates without stabilization, which is to say, they are the segment rates used for calculating the maximum deduction limit. Your premium funding target using the alternative method is just your (vested) maximum funding target.
Good question about the lookback month, it doesn't come up very often that a plan changes its lookback month. My guess is that if you changed the lookback month for funding purposes, it would also change for alternative premium funding target purposes.
Remember that the segment rates for the alternative method are 24-month averages, so they will tend to be higher (=lower premiums) when interest rates are falling, and lower (=higher premiums) when interest rates are rising, as compared to the standard method (spot) rates. With interest rates looking to rise sharply this year, I would be very careful about recommending a sponsor switch to the alternative method now, since it will lock them in to the 24-month average rates for 5 years.