The one thing that people seem to be ignoring here with the whole welding thing, is that even in the TNG tech manual, there was mention, in the history of the Enterprise D's construction, of the first parts of the structural skeleton of the vessel being "Gamma Welded" together.
Even if they DO use something of the like in the film, which I doubt, modern welders are designed the way they are for a reason, in order to make it as usable as possible. Perhaps they only happen to LOOK like modern welding devices, because a welder is designed in the best way they could think of to weld something, at least, given modern technological constraints. We can't even guess as to what the actual technological background of the device, so how do we know that it isn't some form of advanced (insert technobabble)welding device.
As for the sparks? ANY process which can fuse two pieces of metal together to make one is almost certain to involve a fairly large amount of energy, a good bit of which is most likely going to be wasted. This energy needs to disperse somehow, so who says it isn't in the form of sparks?
As for the ribbing on the nacelles, as some people interpret it, if you look closely, you can see a scaffolding running along the side of each nacelle. These scaffolds have a number of lights on them in a row, most clearly visible on the starboard nacelle. These appear to each cast a relatively localized light, and create a strong highlight where their reflection and bloom appears on the hull.