simonsays, 20A is a disused word for a plant "term for spinach = H. The answer is longer than the entry.
36A = Proper name of a US naval officer, that means extremely and jolly as in 'it's jolly good'.
I can't help with 25d and 29d as I'm stuck on those too.
I'm now finding this very tedious. There are so many variables as it's difficult to determine which the replaced letter is in some cases, plus letters that replace themselves, resulting in greater ambiguity.19d appears to be one such, so the letter could be any one of six. A second could be 30A or 31D, but which?
On top of that, the pairs of letters taken in clue order don't seem to yield anything sensible.