I agree with your opinion that DMRs are underpowered, and I also agree with your suggestions on how to fix DMRs, but I'd still like to offer a few small corrections.
Time-to-Kill Comparison:
This isn't exactly a fair comparison. You're using the close range TTK of SMGs and Carbines, while not properly taking into account the fact that those weapons have significant damage drop off, while the DMRs have damage drop off, it doesn't affect their TTK or bullets to kill. This means that beyond 50-80 meters DMRs have better TTK values. There are obviously other things to consider, but it does make more sense if you start comparing weapons at the distance they are supposed to be used at.
Most DMRs use the standard 1.34x headshot multiplier.
This is incorrect, most DMRs use the 1.5x headshot multiplier except the LMR27, which presumably because of its high fire rate only has a 1.34x multiplier. For the M39 and SVDM, headshots reduce the BTK, but only in case you combine one headshot with one body shot. Increasing the headshot multiplier beyond 1.5x, to 1.75x or 2x doesn't fix the TTK in most circumstances. The exception is the SVK, which does benefit from a multiplier of 1.75x or higher, as that would extend the one shot headshot range.
