Wow! Thank you sooo much!
I didn't expect such a detailed reply!
You are correct in that in the real world the distinction between the two 'meanings' is not that great and could be negligible. But actually I believe that in (a) and (c) there's a grammatical difference... or maybe it is a semantic difference.. I'm not sure what to call it... Maybe the difference doesn't really change things since the same goal can be attained by two different methods. There are two different ways of interpreting (a) and (c) although the end result might be the same!
The 'quite' in (b) however makes things quite clear.
Maybe it is just semantics. One could argue that 'so' and 'that' could have two meanings, and therefore the question is a semantic question. But maybe at certain points semantics cannot be separated from grammar.
It is an interesting question.