Finally finished, what a struggle! A true challenge from Kirsanov:-)
Before I file it away, can I get some closure on the parsing for ...
16a - Parcel returned with note to timeless Old Emperor of Japan (6)
I think this is a normal clue so the special instructions don't apply.
The definition is "old emperor of Japan" and I think the last letter could be "to timeless" so O and the penultimate letter could be a "note" but I can't equate my first 4 letters to "parcel" forwards or backwards/'returned' in fact, they don't even make a word (but would if BO was added and they were 'returned')