DMG page 45 (sorry, I don’t have it on D&DBeyond)
To determine whether there is line of sight between two spaces, pick a corner of one space and trace an imaginary line from that corner to any part of another space. If you can trace a line that doesn’t pass through or touch an object or effect that blocks vision - such as a stone wall, a thick curtain, or a dense cloud of fog - then there is line of sight.
There you go, if you can draw an unobstructed line from any corner of a creature’s space to any part of another space, that creature has line of sight to that space.
Yes. Yes it does. Which is why we’ve been trying to tell you the stealth rules are broken.
See above - DMG page 45.