I'm not very convinced by post #5 'expected output waveform', in my ignorance, my very great ignorance, that looks like a bog-standard power supply rectified and smoothed signal more than an envelope detector. Why do you think a signal that should faithfully follow the input should hold up the output voltage? I'm hoping to learn something about envelopes, that's why I ask.
And, surely for it to have no negative level at the output requires a second diode that is reverse-biased?
View attachment 167156