A. Landslide
Skeeve tries to find the linchpin boulder in their way and move it using his Onyx magic, thus releasing enough of the blockage to allow them to cross it more safely. He tries multiple boulders, from different spots. There is a single roll, at disadvantage, to represent the sum result of the entire effort. (These are big rocks, and there may not even be a key rock to release the others. I assume he does successfully remove a few no matter what, but without a successful roll, they just don't matter.) Success means that the way is cleared to allow everyone to pass. Failure means another solution is needed.
B. Cliff
Skeeve ties his two ropes together, and ties off one end. He then uses the length of rope to descend to the entire length. He will try to use the ledges, but uses the rope to make sure nothing really bad happens. The roll here is for the descent. Success means he made it unscathed and unharmed. Failure means he takes a point of damage on the descent.
Once he makes it to the bottom, Blornvid unties the rope so Skeeve can reel it in for another time. He then uses his Onyx magic to carry them down to his location one at a time. Ordinarily this should not require a roll since they within his "free" weight limit, but since all of this is new to him, I provide one roll for each at advantage. I choose advantage because it shouldn't require a roll normally, but does this time. Failure means they take a point of damage.
C. River
In this case Skeeve simply repeats what he, Khaguran, and Rikia did with a much bigger river. He ties off the rope again to a nearby tree, then uses the crossbow to tie off the other end to a tree on the far side. We all scootch across and he "mage hand"s the knot on the far end to retrieve the rope. (If he can't this time, he'll leave the rope.)
If there is nothing to tie off on, he ties both ropes to himself and one rope end to each of the other two. (So, one rope is Skeeve to Blornvid, the other is Skeeve to Gardakhan.) Skeeve uses Onyx magic to put Blornvid on the far side. (Blornvid will hold Skeeve's pack during this.) He then crosses the river the hard way, using the other two as anchors. On the other side, he then uses Onyx magic to bring Gardakhan across.
With the prior lesson, I'm not rolling for the others. The roll is Skeeve's manual crossing. Success means he gets wet, but that's it. Failure means he takes damage while fumbling around during the crossing.
Rolls
Skeeve: Clear Landslide - (1d6)
(6) = 6
Skeeve: Move Blornvid - (3d6)
(664) = 16
Skeeve: Move Gardakhan - (3d6)
(345) = 12
Skeeve: Cross River - (2d6)
(25) = 7