AM Demodulation of Square wave
R8 (1 Mohm) and C4 (15 pF) have a time constant that is too long for decent 10 kHz demodulation. Try lowering R3 to 1 kohm. You will get more carrier ripple coming through but the square wave shape will be much improved.

Regards your modulator, it is a very practical use of a simple diode to achieve AM but your inductor is too small and your capacitor is too big for a practical design to use as a bandpass filter at 800 kHz. Try raising the inductor value by a thousand and lowering C by a thousand, then recompute R4 to be about a kohm. That output filter and its Q is fundamental to optimizing the modulation shape in this type of diode modulator.