Glamorgan rested Colin Ingram after a blow on the hand against Hampshire, with Nicholls and 17-year-old Tom Edwards replacing the South Africa batter and Connor O’Riordan.
Meanwhile, Worcestershire brought in Beyers Swanepoel for Tom Taylor.
Ul Hassan, who had not bowled in the previous match because of a sore shoulder, struck three times in his opening five overs as Isaac Mohammed was bowled driving, Rehaan Edavalath feathered a catch behind and Brett D’Oliveira edged to slip.
Kashif Ali, with a lively 40 off 38, started the revival before he was caught behind off Dan Douthwaite having added 70 with Libby.
Cullen took over the attacking role as he and Libby started strongly, the captain reaching a hard-working 50 off 72 balls while Cullen’s took 60.
Glamorgan rotated their bowlers frequently but could not produce a hint of a breakthrough as Libby accelerated to three figures off 109 balls.
The pair cut loose late in the innings, taking their partnership to 153 as Cullen brought out some mischievous ramps.
But Edwards held his nerve and had both caught as he claimed three late victims, with five wickets falling for 29 in the last five overs.
Libby took his tally to 432 runs in four games for two dismissals after three successive knocks of 109.
Glamorgan made another shaky start with Eddie Byrom caught behind off Ben Allison, Harry Wallace skying a return catch to newcomer George O’Connor and Ul Hassan slashing Ben Gibbon’s first ball to point at 46-3.
Young home-grown batters Hope-Bell and Nicholls steadied the ship and showed welcome promise with 107 on the board at the halfway stage.
Hope-Bell was the more aggressive and struck eight fours with some elegant stroke play in a stand of 119 before he chipped a tame return catch to D’Oliveira.
Alex Horton was run out for nine after a mix-up and Glamorgan were short of wickets, five down and needing 83 in the last 10, even before Nicholls skied Fateh Singh to mid-off trying to up the run-rate.
Gibbon returned to have the powerful Douthwaite caught for 20 on the point boundary.
Van der Gugten, a centurion against Essex on Friday, played his part and clubbed three boundaries in the 48th over, but he was caught at deep point off the last ball from Allison in the 49th.
Gorvin had struggled in the final over against Essex, when Glamorgan could not manage six in the last over, but turned the tables despite needing 12 this time.
Singles came off the first two balls but the captain found the mid-wicket boundary off the third and scampered a couple of twos.
Worcestershire staged lengthy discussions with two needed off the last delivery from Gibbon but, with third man up in the circle and the wicketkeeper standing up, Gorvin (25 not out) held his nerve and carved the ball to the ropes to finish Glamorgan’s campaign with victory.
BBC News