Adding a center speaker, you will then be using all 5 channels of the A51 for HT. Are you then going to disconnect everything and reconnect for bi-amp to do stereo only listening?
When you drive the 803D with a single channel, the majority of the power is being used by the LF input, with a small amount of the power being used by the HF input. So, to bi-amp, you are just adding more potential of power for the HF input that is just not necessary. I agree that more power can sound better at low volumes, however, that extra potential of power needs to be available for the LF input, not just extra power on the HF input.
Something that you may not be aware of is that the A51 will have more output capability when only using 2 channels as opposed to using all 5 channels. Check the measurements here
for 2 channel vs 5 channel.
Is bypassing the b&w crossover easy?
No, as this will require serious disassembly of the speaker since the crossover of the 803D consists of 3 separate boards in different locations inside the speaker.
If I do not end up biamping, should I at least biwire?
This has been debated extensively on this forum with various opinions. IME, I prefer single wire with matching jumpers to replace the factory straps between the LF and HF connections, although I believe the 803D comes with a much better jumper than the typical gold plated brass factory strap. If you bi-wire, I believe most agree that it is best to use 2 separate runs of identical cable.
Regarding centers, I may get a different center as my cabinet isn't tall enough to house the tweeter for the matching center.
Understand, but it will definitely need to be a B&W.