Regarding the first question.
On KB2 and (MC2 non-NBT), it's quicker to finish the last lap way outside the track offroad you can't do this on laps 2-4 because you would slow down in offroad and ruin the next lap. on KB2 this is done with a waterslide into the ocean way right of the finish line. And on MC2 this is done hitting the bumper and staying left of the road close to the wall and also letting go of the gas so the offroad doesn't slow you down as much.  

Donut plains 1 is also a little faster using a relatively simple NBT getting a boost on the last corner and stayng close to the wall on the right side without holding in the gas. this is exatly the same as is done in rDP1 5lap in MKSC which you are familiar with.
I'm not familiar with more advanced NTB strats so there might be more.

there are definitively some tracks where you have an advantage on the flap entering from an angle which is disadvantageous in a 5lap setting. KB2 comes to mind where you wanna be a little more to the left. also if you play non-nbt on tracks where you lose speed and don't cross the finish line at full speed. For flaps you might have to drive extra distances to ensure you have enogh accelration time to get good flaps. and this will of coure make a big discrepancy beetween best times achieved during 5lap attemps and flap attemps.
With NBT you can hold a strategic boost while entering the lap.


But on quite a few tracks (especially non-NBT) like the  BC's and GV's you probably can just time your best lap by 4 and add best starter lap for a very accurate 5lap potential.