Dried out and brittle foam seat padding

GL-1 is straight mineral oil and is not suitable for the EP requirements of hypoid gear differentials.
GL-3 or GL-4 is OK.

The "U" shaped channel is on the front and rear sides only and ends before the upper curve in the door opening.
The top piece is a flat strip of rubber or felt that the top of the glass presses against when the window is raised. There is no "U" shaped channel on the top side.

Looks like trim code 83 or 84, listed on pages XVIII and XIX of the index for the 1951-54 parts list. It just shows as blue or dark gray nylon combined with light gray nylon.
The code should be stamped on the serial number plate.

Remove the pedal pads and toe board then slide the seals down to the stop on the brake and clutch arms

Power seat actuator and motor

The small black unit is an Auto-Lite Point Guard which is intended to reduce excessive current draw during idling and low speed driving according to the Merchandising Items section at the back of their 1954 catalog for service parts. An online search will turn up photos to confirm.
It is an aftermarket item and is not required for the 1951 Packard.

See my response re 1950 gas gauge from December 8 2022 that provides a link to the Dennis Carpenter website for a Ford unit

Check Steele Rubber for convertible roof rail weatherstrip

Quote:

It provides a slight delay in the sound to the rear speaker to give the feeling of a larger space