Tidal effects are not related to the magnetic field. The earth's core is mainly liquid iron and nickel, with a solid iron-nickel inner core. There will be convection and rotation, and therefore dynamo effects even without the moon.

Ditto vulcanity. There would still be lithospheric plates grinding around, and plumes (heat convection) in the mantle, without any help from the moon. (Though when the moon first formed, it was a lot closer to earth, and the tidal rise and fall of the land was of the order of several metres. That must have had a lot to do with cracking up the earth's original proto-crust.)



I can't really accept the theory that the moon has been sweeping up rocks that would otherwise hit earth. Earth's mass is ten times that of the moon and would attract the majority of whatever came the way of the earth-moon binary system.

Also, the moon has two distinct surface types. The lighter is anorthosite, which is over 4 billion years old, and very heavily cratered. The darker is basalt, 2.5 billion years old, and much less heavily cratered (or at least, has much fewer large craters. The surface is pretty well pulverised, by impacts from micrometeorites which earth's atmosphere stops). The inference is that there was a very intense phase of bombardment which accompanied the moon's formation, but since then, there has been a lower rate of random impacts.

Other than micrometeorite impacts, there are no processes on the moon which obliterate surface features. On earth, even a crater on land is slowly worn down by wind, frost and especially rain, and eventually buried to survive only as a trace on a gravimetric survey. (The one at Meteor Crater in Arizona has survived remarkably intact because it is in an area with low rainfall.)



Smashing the moon to pieces would probably lead to a lot of the rubble falling on earth, with overall effects similar to the impact which wiped out the dinosaurs. The remaining debris would probably form a ring system; not as spectacular or as long-lived as that of Saturn, but pretty enough when observed from north or south polar regions.