What options should I use with FFmpeg when I'm downmixing DTS 5.1 audio into 2 channel audio?

ffmpeg -i inputfile.dts -ac 2 -acodec pcm_s32le -ar 48000 -f wav -request_channels 2 - | neroAacEnc -if - -q 0.4 -lc -ignorelength -of output.mp4
- Should I use pcm_s16le or pcm_s32le?

- Is -ar option relevant? The source already is 48 kHz.

- What does -request_channels option do here? I already have -ac 2 option to specify for 2 channel downmix. As far as I understand it, the request_channels option reads metadata from the input somehow?

- How can I normalize the audio after downmixing (before encoding)?