I thought bombardment was the way to go - however, I'm liking Ville's natural (no power supply needed) magnets - assuming the science is correct.
I don't buy thermoelectric powerplants lasting for millions of years. OTOH, if they're miles below the surface, Martians are unlikely to run into the robots need for irregular maintenance, etc. But natural magnets would be superior, even if they're a much larger engineering project.
You're going to have a significant cooldown period. But you're planning on evolving races anyways, so you're going to be waiting around a long time anyways.
Plus side? You can get the spin up to where you want, and axial-tilt with judicious bombardment. As well as add water. Possibly oxygen if you disassociate the water, and migrate out the hydrogen. You can also monkey with gravity with a sufficient amount of mass added (non-trivial).
It's probably going to take a significant amount/number of precisely timed strikes to get what you want going on done. Which is going to be computer guided, possibly machine constructed engines, and a fair amount of delta-V (but, over long enough time periods this isn't all super-hard - and given a longer timeframe, you're definitely going to use gravity-assisted slingshots; perhaps even lengthening Jupiter's rotational period by stealing energy from it).
Bombard the [heck] out of it, add iron, melt with that added impact energy, rotate to get a spin going. Then wait to cooldown. Maybe take out hydrogen while you don't have a lot of liquid running around. Seed (since you don't want to wait) with some stuff, depending on how virgin you want your evolution - you could skip the hard parts that took us billions of years, and get right to business. By giving them chloroplasts, mitochondria, and multi-cellular life. More, if you want more. But too much, and you might as well be bioengineering them instead of 'evolving' them.
I'd want some math geeks to chime in with how long it'd take to cool, and other things. You might (unlikely) be able to get it cooled down in 1000s of years. But I think hundreds of thousands, or millions might be more reasonable; ie: WAGing here.
Also bombarding with really fast asteroid can change Mars orbit.
This is why I said you're going to need a number of different impacts. You'll also (most likely) want to distribute the heat and energy to melt things more evenly.
Also, you'll want to avoid shearing off a hunk of Mars creating a major moon, like the Moon.
How many? That's a good question. Faster they are going, the fewer you'll need. But the fewer you have, the less you'll increase gravity by. A non-trivial amount, for sure. We need Math for that answer.
WAG: Mars == 1/10th of Earth's mass.
7.04377e20 short tons. If I'm not screwing up my exponents: 704,377,000,000,000,000,000 tons.
Let's WAG it at minimum of 1/1,000,000,000th of that. :)
The other option is generate a lot of power (A LOT); probably fusion or matter/antimatter, inside the core(well, more likely the mantle: drilling deep enough is also a non-trivial matter, also requiring a lot of power - but perhaps you can just run plasma-beams to gasify all matter in your way until you get to where you want to go), and then induce a spin in it with electromagnets in space (which will also require A LOT of power). And that's assuming there's enough iron in there to make things happy. You'd need to lower water (maybe small enough chunks will melt in the near-nonexistent atmosphere on re-entry; making that many small chunks of ice is another exercise left to the reader) if you don't want an Arrakis. Frankly, that's a lot of machinery and power you're going to have to generate, merely to attempt to shave off a few hundred thousand to millions of years of cooldown from a molten state. Might be cheaper energy-wise to space-elevator in some refrigerators to pump heat into orbit to make your cooldown from a molten state quicker, if you're that concerned about the time it might take.
1,260,000,000,000,000,000,000 liters of water in Earth's ocean. Assume 1/10th of that for a mass 1/10th of Earth (although is probably less than that).
Assume you can get 1 liter of water to vaporize in the atmosphere (I think that may be too large) upon re-entry. That's... a few chunks that you've got to separate and get into the atmosphere. Probably need to build an auto-melter that makes snowballs from a larger chunk, and drops them down the gravity well.
On the plus side, if you're going for Molten-Mars, these ice chunks can be bigger, and (perhaps? impact adds heat... but how much?) used as a (very minor) part of cooling down the planet.