1] Transformer - unless it has a center tap to ground the hot and cold aren't ground referenced.
Because you have an RCA-only power amp without a bridging switch?
I never saw this until now, but yes, and that is exactly how I bridged several power amplifiers many years ago. I created two signals 180 degrees out of phase (inverted) and drove two channels, taking the speaker output across the (+) terminals of the two amplifier channels (or two mono amplifiers). A simple pair of op-amps was good enough for a subwoofer, but for a more sensitive full-range application I used a more complex circuit to create a truly differential signal from the single-ended input. I have also used a transformer, but good transformers are expensive. Getting high CMRR (common-mode rejection ratio) takes careful design and layout.
