[netllama]

You've disabled use of the EDID:
(**) NVIDIA(0): Option "UseEDID" "FALSE"

As a result, no modes can be validated against the EDID.

However, here's why your custom mode is failing to work:
(II) NVIDIA(1): Validating Mode "1280x720@50":
(II) NVIDIA(1): 1280 x 720 @ 50 Hz
(II) NVIDIA(1): Mode Source: X Configuration file ModeLine
(II) NVIDIA(1): Pixel Clock : 62.16 MHz
(II) NVIDIA(1): HRes, HSyncStart : 1280, 1320
(II) NVIDIA(1): HSyncEnd, HTotal : 1488, 1640
(II) NVIDIA(1): VRes, VSyncStart : 720, 722
(II) NVIDIA(1): VSyncEnd, VTotal : 723, 758
(II) NVIDIA(1): H/V Polarity : +/+
(WW) NVIDIA(1): Mode is rejected: Mode (1280 x 720) is too large for DFP
(WW) NVIDIA(1): Native Resolution (Max: 640 x 480).