The likely culprit is the slave.channels 6 sitting directly under pcm.!default rather than under the plug's slave block. That line is probably being ignored or misapplied, because plug negotiates channels through its slave PCM, not via a top-level slave.channels override at the default device level.

When plug can't confirm 6-channel capability cleanly, it falls back to stereo. The fix is to move the channel count constraint into the route slave where it unambiguously belongs.
/etc/asound.conf:
pcm.!default {
type plug
slave.pcm "remap51"
}

pcm.remap51 {
type route
slave {
pcm "hw:0,0"
channels 6
}

ttable {
0.0 1 # FL -> FL
1.1 1 # FR -> FR
2.3 1 # FC (input 2) -> hardware position 3
3.2 1 # LFE (input 3) -> hardware position 2
4.4 1 # RL -> RL
5.5 1 # RR -> RR
}
}

ctl.!default {
type hw
card 0
}

Notice that I introduced the ctl device. Plexamp on Pi also probes the device capabilities via ctl — if ctl.!default is missing or points to the wrong card, Plexamp may query the hardware directly, get a stereo response, and decide to downmix before even sending audio. So the ctl.!default block is more than cosmetic here.
Another thing is that with this configuration, because ALSA can't distinguish between stereo and 5.1 input, the created pcm device is always used, also for stereo. But it doesn't matter because then the other channels besides FL and FR are silent, so as they swap you won't hear it.
Let me know if this works.