Why? Olympus Mons is a shield volcano, they basically only let out lava and barely any gasses at all. You're skipping a bit with the dynamo thing, and while that is how a planet creates a magnetic sphere, there needs to be movement, electric current and a magnetic sphere (having two makes the next one etc etc I'm assuming you know this).
Earth has also had 'several different magnetic fields' I don't think you quite understand my point altogether. Earth has them and the flip around every few million years, and we can tell because of these things called magnetic stripes on the sea floor. This doesn't mean Earth's magnetic field, and the core for that matter, has suddenly 'stopped working'. Earth's core remains hot due to the energy released in radioactive decay, hence the outer core remains molten and acts as our 'movement'. Electricity is formed through friction, and so the magnetic field is created.
On Mars, there is no magnetic field. This is because, we assume, the outer core does not have enough energy to remain molten, or the energy being created isn't enough to do so, or many other reasons which are besides the point. Radioactive decay simply happens. It doesn't stop and start, it occurs regardless. So, if Mars does not have a magnetic field now, it probably never will. Cores don't start and stop, their energy comes from something that is either happening, or isn't.
Olympus Mons isn't going to destroy any terraforming even if it could, because it's like Hawaii's Kīlauea, which just slowly releases lava and not a whole lot else. The reason it's big (which I'm assuming is why you're comparing it to a super volcano, which is a whole different beast) is because Mar's has no plate tectonics, so while hawaii's an island chain, if we had none then there would be no chain, just a huge volcano.
The core won't reactivate due to the pressure (unless, of course, you are putting a -lot- more pressure on it than is occurring now) because that isn't where the heat comes from. If it was, then it would continue to forever -have- a molten core because the pressure is constant and you would have energy forever, which isn't conceivably possible.
Other volcano's on Mars? Maybe, but again, they have to get that heat from somewhere, and if isn't coming from the core, where do you think it is coming from?