No short lines do not kill upwind ability. Going upwind is a balance of power (having as much as you can) and directing that power. The best way to get power is by opening the rig and forward then balancing the sail in it's sweet spot (going upwind raking the rig back is best direction of power). If your short lines are killing the power it is not a problem with the line length but a problem with you having the lines too far back so that you are oversheeting, killing the power and not having the drive upwind.
The one problem with short lines is larger board widths effectively makes you sheet in more so you are driving the power into the wrong place.
I used short (18") lines for many years and have always used a high boom and am not that tall at a smidge under 6 foot. Looking at photos of times gone by oversheeting whilst upwind was a sometimes a problem mostly when sailing into lulls they don't give you that reaction time a longer line does. Bending your arms is about technique not line length- if you are bending your arms with short lines whilst going upwind you are not leaning forward (which naturally rakes the sail back).
Regarding Formula, long lines help get the rig away and stop sheeting in over the centreline thus directing power the wrong way again. Upwind I did shorten lines a bit and never had any issues matching guys for speed and pointing who where far more experienced than me on formula.
Regarding Waveboards, I use 26" fixed lines and waist harness. I tend not to rake the rig back when going upwind but stand up and use the power of the rig and direct it into the right spot- raking the rig back on a waveboard loads the fin too much and kills your ankles. If I really need to point high I will rake the rig back, but take back foot out of strap and lean forward (bring back robby's second back strap

). Its just easier to sail around in that sweet spot though.
For slalom and speed I use 22-30" lines and seat harness with it set most of time around 24 for upwind and 28 for reaching.
People need to adapt their technique, not look for quick fixes in something like line length. Short lines you tend to lean back alot to counter that rig, longer lines you need to consciously stand up straighter to maintain that 7 stance.