Great questions.
1. The pass band of filters inside the onboard chip in the analog section of the NI USRP-2921 are visible for very wide spans. It's flat for short spans, but as you can see the curvature on a very wide span such as 25MHz. Only about 80% of the bandwidth is usable due to filter edge effects, so your real-time bandwidth is closer to 20 MHz. The 2921 does include adjustable baseband filters for Recieve at fixed values of 15M, 19M, 28M, 36MHz that can be accessed from the niUSRP property node titled Bandwidth. (the curvature you are seeing could also be caused by the antenna, but this can easily be ruled out by simply removing the antenna.
2. The USRP is always sampling at 100 MS/s on I and Q giving a theoretical realtime bandidth of 80MHz. Because of the onboard analog filters reduced this to about 40MHz and down-sampling done digitally mathch the data rate to that of gigabyte ethernet, making 25MHz is the widest possible bandwidth for 16bit samples. The property node you are using to change frequency on the fly allows you to move around within the 40MHz window without re-tuning the LO (local oscillator). In order to move outside of that 40 MHz bandwidth you would need to stop the Rx task, retune the LO center frequency, and restart the task.
