Confusion regarding considering a transmission line as lumped or distributed
The lumped model of a wire (or any other element) is an approximation. It's not entirely accurate, but it can be useful in many circumstances. 

The distributed transmission line model given by the telegrapher's equations is another approximation. It's useful in some circumstances where the lumped model becomes very inaccurate. But it will itself break down and be inaccurate in some cases.

If you are worried that the lumped approximation might not be accurate enough in your situation, you should solve your problem both ways (or maybe a small piece of your problem that will reveal the scale of the discrepancy between the models). If the difference is big enough to affect the final conclusions you are trying to draw from the model, then you should use the transmission line model. If the differences are too small to affect your conclusions, then you may be able to save some time and effort by using the lumped-element model.