Never been behind a unimog, but here it is :

-the dash throttle lever should be connected to the pedal linkage



Check the connection, alter the length of the rod #48 by the ball sockets. Worn bushing(s) in the assembly would give you a sloppy feel and in your case the slop may be the reason for failing to achieve the "stop" point.
Also pulling the gas pedal back may shut the engine off, so try this too.

Check how operating the hand throttle affects the gas pedal position :

https://youtu.be/dxJX8j4adD8?t=13

The repair kit for the lift pump should be available from Peachparts, #0015861207

https://mb-teilekatalog.info/view_SubGroupAction.php?lang=E&mode=BM&class=6&aggtyp=M&catalog=172&model=616911&group=07&subgrp=045

Please do NOT swap the LP for an electric one ! No need to buy a new one either.

Pretty much everything concerning the injection system and the engine is covered almost completely in the forum, so search the archives (easy) or ask straight.