Is the 1.5 degree celsius warming locked-in, even if we reduce carbon emissions to zero tomorrow?

Yes, for a variety of reasons. First and more importantly, let's get real. It seems highly unlikely that the drastic emissions reductions required to stabilize the climate to 1.5 will be palatable to the governments of the high carbon emitting countries. Second, if we ever get to zero carbon emissions, we would also have zero sulfate aerosol emissions. These particles mask about 0.3C of warming that we would otherwise already have. This is partly offset by zero methane emissions that would allow existing methane to gradually oxidize into less potent carbon dioxide. And of course, the ocean time scales are long and will continue to warm by a few tenths of a degree C after zero emissions. But as the planet is already at 1.1C above preindustrial temperatures, I see no practical way to limit global warming to 1.5C