Just as the FAQ said, my driver footwell was rotted out due to one thing only, a small leak from the vent plug covering the optional European non-A/C hole for a manual vent to cool the driver's feet. Water was entering the air grille intake under the windshield as per normal, and while most of it passed through to the rocker panel and out, some crept past the seal around this vent plug. You get at it by taking the left side kick panel off. It took a claw hammer eventually, with some damage to the plug.

When I was able to see in there, I did the bucket of water test. Yep, it shot right in onto the floor. The rubber tube coming past that area (assume it collects cowl water and dumps it) was only showing minor flow, while major water was coming from above, unchanneled. Maybe my tube intake is clogged. Anyway, I put the plug back with plenty of 3M Window-Weld Ribbon Sealer (for replacing glass - very sticky, Part No. 051135-08612, $11 from Advance Auto). The original leak was a tiny trickle, but resulted in huge rust holes. Worth checking carefully.

By the way, why would the Volvo designers dump all that water into the rocker panel (classic rust trap, depending on simple, cloggable slits to drain) instead of leading the tube (only 5 more inches) out of the body shell, as they do for the miniscule amount that gets into the radio antenna?