What happens to a DSB signal if I use an envelope detector instead of Low Pass Filter
What you call envelope detector, consists of the detection diode and a low-pass filter. This is one way of demodulating an AM signal, more precisely, it is called asynchronous demodulation.

If you do not connect the diode, and only you process the AM signal with a low pass filter, you get the modulating signal, to which is added the carrier signal. Watch the third oscillogram well. It is the component of the modulating signal to the signal shaping, while is added a proportion of the carrier signal, attenuated by the filter.

The correct operation, is to use the envelope detector. The problem we are having is that the modulating signal is too high (amplitude), so when you make the detection and filtering, unwanted components appear, causing the distortion you see in the second oscillogram.

In the first oscillogram, one can see that the modulation index is extremely high; you should decrease the amplitude of the modulating signal and use the envelope detector to demodulate the transmission.