Volvo realised the design wasn't ideal and produced an aftermarket accessory called a 'snowcap' which clips over the vents in an effort to allow airflow in but keep leaves and other debris out. The only way to be really sure they're clear is regular cleaning via the various plugs though.
I have wondered occasionally if they could be cleared out effectively with a garden hose pushed into the plug in the rear wheel inner arch, but haven't got around to experimenting. Yes it'd fill the sills with water, but they're going to be damp anyway as soon as it rains and it might work as a way of flushing all the associated debris towards the drain holes - which I'd be frantically clearing with a piece of coat hanger or similar as soon as the hose got turned on....
Unless the outer sill was really bad I'd probably try to let in new metal and patch it with carefully fitted repair panels butt welded into place. The section under the B-pillar would be hard to extract and replace cleanly I suspect, although of course it has been done by others at home on the drive.
If you're going to replace the whole sill I'd try to use a section of original metal as a guide and form the new sill into as close a profile as you can make it before attempting to attach it to the car.
Cheers
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