In a completely hypothetical scenario, say both of the Soyuz vehicles docked to the station were damaged/destroyed, but there was a Dragon 1.0 on the station left intact. Although i don't believe it has proper life support, if astronauts had spacesuits on with enough oxygen and CO2 scrubbing capability for the several hour journey back to Earth, would Dragon 1.0 be a suitable return vehicle?
It's better than nothing, of course. I don't think it has carbon scrubbers but other than that it's climate controlled and pressurized (Elon: "If someone were to stowaway on the unmanned version of Dragon, they would have been fine"), and is designed to safely return payloads. It's probably survivable as is, and I'd say they'd have a pretty good shot of making it back.
Stealing equations from here, since Dragon has 10 cubic meters of pressurized space, 0.3 cubic meters of CO2 would be toxic. Each person produces about 0.036 cubic meters of CO2 per day. So one person would have a week of breathing air, and even three people could survive over two days. CO2 scrubbers, while desirable (CO2 will reach dangerous levels within a day), are probably not necessary, especially since if a rescue mission isn't viable they're probably returning as fast as possible.
If necessary, spacesuits could be worn adding at least 8.5 man-hours per EMU. Sokol flight suits can be worn for 30 hours in a pressurized environment, but I'm not sure this means 30 hours without external life support. Neither of these have carbon scrubbers, so all crew would have to don these at the same time when CO2 in the capsule starts getting too high. Not sure whether ISS scrubbers could feasible - Those were definitely not designed to fit in a capsule.
The other big challenge is surviving impact. 3.5 g, while uncomfortable for sure, is not going to be lethal unless something goes really wrong, and if the Soyuz spacecraft are merely disabled, then acceleration couches (among other things) can be salvaged. The impact of hitting the ocean is probably more dangerous, and I frankly can't find figures on that. Without couches, serious injury is likely, especially if the crew is in suits.