I have a Pioneer that fluctuates between 90-98% during the burn. I wouldn't worry about it as long as it's not the main buffer that is lower than 100%.
The 7261 is the same drive as the 7260. Only difference is that the 7261 has light scribe and also usually costs a bit more for that feature.
Same goes for the 5260 that is the 7260 except DVD-RAM burning support. Usually the cheapest model.
You can't disabled it. Instead you should actually pick a location there the layer break is noticed less.
You might try the Clone mode in DVDFab and create the ISO within that program. It should preserve the layer break.