"if life ever developed on Mars, and if anything survived the changes in the planet's ecosphere over the aeons, the most likely place to find it would be in that ice."
I think Valles Marineris would be a better bet for life if it exists/existed on Mars.
Formed as Mars cooled, the depth (up to 7Km), should yield a denser atmosphere, plus its alignment (East-West), depth*, and location (Equator), should presumably be warmer than at the surface altitudes.
No doubt it contains caves and overhangs which could harbor residual ice, and maybe even seasonally temporary pools of liquid water.
Unlikely, but we'll never know for certain until we look.
*Mars is too cold for a molten core, but it still retains some residual heat.