Re: Mahagony decking maintenance
We use mahogany for porch flooring, but not in a fully-exposed deck installation. In your case you have a rather extreme exposure with snow, salt water/mist, and hi UV. I would only consider IPE for a situation like yours. And just let it weather. IPE turns a very gentle gray in about 2 years, just like cedar shingles. For reference, see the boardwalk at Atlantic City.
All of the factors against lesser woods are also brought to bear against any type of clear finish or stain that you would strive to maintain; a losing proposition I'm afraid unless applied yearly.
Remove not the ancient landmark, which your fathers have set.