Not an expert, so take these with a grain of salt:
The core of the star is the hottest and most dense part. Greater heat and density make it easier for fusion reactions to run. If suddenly the core is made mostly of iron, then the amount of energy it produces rapidly drops. Even if there are nice, easily fusible hydrogen atoms farther out from the core, they will not be fusing at a very high rate, because the temperature and pressure is lower where they are. Also, the more easily fusible atoms remaining outside the core can't diffuse into the core fast enough to refuel it. The only possible outcome is collapse.
In some sense "dark matter" and "dark energy" are just placeholder words for "whatever thing is causing all this weird stuff to happen". This is actually very analogous to how "the ether" was a placeholder term for "whatever thing that radio waves are waves in". (Now we refer to it as "the electromagnetic field". The "ether" terminology was associated with some incorrect assumptions, such as a privileged reference frame, which is why people sometimes say it was an incorrect hypothesis. But the electromagnetic field is certainly real, it just didn't turn out to work like some people thought it did.) Scientists have observed so far the dark matter seems to behave pretty much like ordinary matter, except that it just happens to ignore the electromagnetic and strong nuclear forces. Not only does it hold galaxies together, but its gravity also bends the paths of light rays, just as we expect of anything massive. So calling it "matter" isn't too much of a stretch. It's still very mysterious, though.
Radiation pressure actually does limit the mass of stars, to something on the order of 100 to 200 solar masses, see this stack exchange question: https://astronomy.stackexchange.com/questions/328/is-there-a... That doesn't stop smaller clouds of gas from collapsing to form smaller stars, though.