Everyone is trained in maintenance of Starfleet Tech but this was non-Starfleet old-fashioned nuclear technology. Uhura is a technician, and it may be that her skills with electronics were what were actually needed but the fact that the engineer couldn't go just nudges my ability to suspend disbelief over the line.It's Star Trek. Everyone is a jack of the trade needed. I can imagine Spock, Uhura and La'An have some engineering training.
She called him and he came from where ever his was in the earlier scene.
Potato Potahto. We know the Kerkovian serum makes deep changes on the psychological and emotional level as well as the physiological level. When Spock went full human he lost all of his Vulcan training and control (He also got a hairstyle change) Kervokians move in mysterious ways.
Suppressing existing DNA by making epigenetic changes is silly but I can see some vague scientific basis for it. This is not the same as overwriting the DNA of one species with the DNA of another species. Vulcan's blood is copper based. Even if the serum over-writes DNA, surely they would need emergency transfusions to avoid multiple organ failure? There are just too many magically hand-waved problems for me. Sorry!