Should be fixed.

Use -max_muxing_queue_size 512 before -c copy.

Forever, because there is @pause at end, just press any key.

If you use libavfilter, you can call av_frame_get_buffer()
You first need to allocate AVFrame with function from libavutil and set some parameters in newly allocated AVFrame, as explained in doxy.
The function to allocate AVFrame is av_frame_alloc().

That message is just informal one, and should not have any impact on output.

Ah, the only way is to use aformat=channel_layouts=mono for every input before amerge:
Repeat similar for rest of tracks.

Use: for each output.
If this still does not work, post FULL UNCUT ffmpeg command AND output.

Use aformat=channel_layouts=stereo after each amerge.

If you ever had read documentation of hstack filter, you would notice that it needs number of inputs, by default that number is 2.

In documentation it is clearly stated that weights are separated with space, and not with '|'.