"Could it be possible to fine tune the capacitor to properly detect a different duration between the upload reset signal and the Serial Monitor duration?
Or are they the same length?"Don't know, never tried something like that or looked at the signal.
I know now 
( If you're interested , search for my "Reset via USB" thread in the "Troubleshooting Arduino" forum )
The duration how long DTR is down, does not matter at all. It's the falling edge of that signal, which ( in conjunction with the internal 100nF in series + 10k Reset pullup) triggers a pulse of about 1 ms.
If you cut the trace, then DTR is no longer connected, requiring manual intervention with the Reset Button to download sketches, not that big a deal.
...
You can later go back and solder the 2 pads together if you want to use DTR again.
It's definitely the wrong forum, but I'm so proud of successfully solving "not that big a deal": I soldered a 2.2k resistor across the pads after cutting.
Now it still resets via DTR, but it's much easier to control this: any resistor around 1k between 5V and RESET inhibits the reset.
One can even find a software solution like
digitalWrite(DONT_RESET_NOW, HIGH);
requiring only little wiring (capacitor, resistor, diode) between that signal, GND and Reset