Even assuming duration boosters apply independently, only caring about the base duration:
Shell (20s) + Buff mastery (10s) + S rank Mechanic card (3s) + shell duration hyper skill (5s) = 38 seconds of immortality.
And for cooldown:
Shell (90s) - S rank Mercedes card (3.6s) - cooldown cutting hyper skill (45s) = 41.4 seconds of cooldown.
The two together make for a
massivereally freakin' small 3.4 seconds of vulnerability between shells. (or 5.2s if the 4% Mercedes card applies after the hyper reduction)