ive been trying both options with the use of a variable delta and the other way of putting interval adjustment code into the "if" that detects button 6, but no further forward.
Adding the below code does not work (it flashes at button6 press but no change in speed on next press of button) but is it close to what you were thinking? I'm guessing the && is the reason along with my lack of my knowledge at this moment in time!
No laughing at my (not so) genius coding knowledge! 
if (results.value == button6 && interval +100)