I saw this post and feel like contributing, since as a musician I've always paid close attention to these aspects.
She never pretended to play guitar, she often played the same notes Monte Pittman was playing but with a different preset, so her guitar sounds different. And obviously her guitar was lower than Pittman's.
Even in Burning Up at the RHT she was playing and you could distinctly hear the sound of her guitar, in fact once for a technical problem she had entered late with her guitar and her guitar was also loud.
Her band can always be heard too, she always had great musicians who mixed perfectly with the recorded sounds.
On some occasions her guitar is actually not so loud in the mix, but if you pay attention you always hear it: even when she makes mistakes you notice that.
As for the piano, I don't know, I haven't seen her live at the Madame X Tour and I've only seen the DVD.
But since she played three very simple chords that even a five year old could learn in a minute, I highly doubt that she was faking it or that she could miss even one note.
More than anything else it was ridiculous that they brought the piano on stage to make her play three chords, when in the rest of the concert you can feel the absence of a band.
