Assault pontoons were separate landing units

Were some pontoons meant to be sunk or grounded deliberately to anchor a long pontoon bridge?

No. The stated plan used separate pontoons attached to monitors, rather than a continuous pontoon bridge.

The intended system used separate pontoons hooked to monitors, rather than a long pontoon bridge. Three, possibly four, are recalled, with six Lord Clive-class monitors assigned to the operation. The diagram is described as placing a ramp ahead of two male tanks and a female tank. Two long trestle routes allowed troops to move forward along the pontoon.