I've logged the buffer and window point at certain lines (most important is
before/after erase-buffer on line 1911) in a few run throughs of
proced-update, first when the selected window displays a proced buffer:

before update: window point: 20121, buffer point: 20121
after goto-char min: window point: 1, buffer point: 1
before erase buffer: window point: 1, buffer point: 1
after erase buffer: window point: 1, buffer point: 1
after update: window point: 20121, buffer point: 20121

So this is expected since buffer point always mirrors window point for the
selected window's displayed buffer.  Next, when the selected window doesn't
display a proced buffer, but there does exist a window displaying a proced
buffer (the window point logged corresponds to the window point of the
window containing the proced buffer):

before update: window point: 20235, buffer point: 20235
after goto-char min: window point: 20235, buffer point: 1
before erase buffer: window point: 20235, buffer point: 1
after erase buffer: window point: 1, buffer point: 1
after update: window point: 1, buffer point: 20235

So my understanding is since the selected window does not display a proced
buffer, the window point is not updated in line with the buffer point, but
the erase-buffer call sets the window point to start of the buffer, and so
this is not updated in line with the buffer point in the subsequent
insertion of processes.

The last case (the second issue) where no window shows a proced buffer is
similar to the previous, but erase buffer instead appears to set pos for
the proced buffer's value in (window-prev-buffers) if it's the case a
window has shown a proced buffer previously.