One difficulty with the Raptors as the launch abort system is that the vehicle would have to be maintained at launch status. Probably off the header tanks. So they'd load those first, pressurize them, chill the engines, and then perform normal propellant load. They'd have to be careful about how many engines they fired, depending on the propellant load.
If anything happens during load of the header tanks, well...
Then there's the issue of thrust-weight ratio once fully prop-loaded. A Starship is going to mass perhaps 1500 tons by the time they're up to the nine-engine variant, and that's less than a 2:1 thrust:weight ratio. Is that a viable abort level?
I’m not sure what happens when vacuum Raptors are fired at full power at sea level; maybe part of their bells could detach in such a scenario to make them more sea-level-like?
They could go with an expanding nozzle. It would allow the engines to be fired as sea level or vacuum variants. Here's an image from the Pratt & Whitney XLR-129 engine - which was never built. There are other
altitude-compensating nozzles, but I'm not sure any of them could be applied to Raptors.
To be honest, I'd go with a completely different structure for this Starship. I'd stack the crew module on top of a Starship thrust section, and include an abort system in that crew module. The only purpose to this vehicle is to get a lot of people to LEO. Once on orbit, they can transfer to a long duration Starship or any other vehicle appropriate to their mission.
So this vehicle becomes a LEO Shuttle. This variant would sacrifice some mass to the launch abort system, but who cares? The only purpose of the vehicle would be to get the crew to orbit safely.
Another alternative is to give each crewman an ejection capsule. Again, the only purpose of this vehicle is to get crew to LEO safely. They can generously apportion that 100-150 ton capacity to the capsules. Done right, those capsules could become a standard part of any orbital vehicle. A little delta-V (solid rocket motor, unpleasant but effective), a little life support, heat shield, cross-range and parachute landing. Or perhaps even a winged belly landing. Single use things. The downside is that if there are 100 capsules with 10 onboard systems each, that's 1000 points of failure that could take a life - or cause an emergency on their own. The design would have to be brain-dead simple and foolproof.