If I recall correctly, Venus has an extremely weak magnetic field, so without such a shield there's a lot more radiation and the water was broken down relatively quickly and the hydrogen escaped into space.

The earth's magnetic field is very strong, making it unlikely that most of our water will disappear, and the sequence you discribe will likely be different on our planet.

Yes, it will take longer (might take longer than the sun will last..) - but that's not a great comfort if you happen to be on the surface.. Indeed the additional effect could see temperatures high enough to melt wet granite.

how much different?