(1a) The southernmost point of England is "The Lizard". — I know it's
correct
Yes.
(1b) The most southern point of England is "The Lizard". — I know it's
incorrect
Why is this incorrect? I would say "not best," but not incorrect English.
(1c) The most southerly point of England is "The Lizard". — Is it
correct?
This is incorrect or close to it. Worse than 1b.
(2a) The kiwi is the southernmost bird in the ostrich family. — Is it
correct?
The bird itself is not southernmost, the geographical distribution or range of the bird is southernmost. However, this is a bit pedantic, and it is correct.
(2b) The kiwi is the most southern bird in the ostrich family. — I
know it's correct
Yes.
(2c) The kiwi is the most southerly bird in the ostrich family. — Is
it correct?
It's close to incorrect, because it sounds bad.
What is the difference between "southernmost", "most southern" and
"most southerly"?
Not much. They're synonyms. But:
Southernmost:
Often used with the phrase "the southernmost point" specifically. Often used to describe a single thing as opposed to a group of things. Example: "southernmost towns" sounds weird, most southern towns is fine, and southernmost town is fine
Most southern:
Good for general use. OK with groups of things. Example: most southern towns sounds normal, most southern towns also sounds normal.
Southerly:
Use this when describing a ship at sea, winds, or the direction of south as opposed to the location of south. If you use this, you may sound artistic or anachronous, depending on how you use it and in what context.