Chinese elm can be a bit cantankerous. Occasionally entire branches just decide to die for apparent reason. Not saying that is what's happening here. That branch may just be slow or may be putting in a protest about the wiring.
It also appears to be the lowest branch on the tree. Not unusual for lower branches to stall or die when the upper parts are allowed to grow freely. The tree still believes it has a chance of being the tallest tree in the forest when upper branches are growing well and lower branches have no use on tallest tree so they just don't bother. In this case trimming all upper areas can help remind the tree that lower branches may be useful.
Bonsai can be, and are, grown in all types of soil mix provided care matches the soil type. Most growers use a more open type of soil mix in pots as it is usually more effective and safer. To change soil you really need to remove as much of the old as possible before potting in the new mix. Combining different types often causes problems. Complete repot in late spring is probably NOT the best timing though Chinese elm have a rep for coping with repot any time of year. I'd just take extra care through this summer and repot early next spring.
If the small pot causes problems with managing water or other care through summer try placing the whole pot, as is, into a larger pot of potting soil. Roots will grow out into the new soil which will give extra water and space for this season but not cause some of the issues associated with slip potting into different soil types.