rtxxx: There are 9 across omitted letters and 3 down
baretter:
33a: The wordplay is a 3-letter tree plus a 1-letter equivalent for AND with the answer, as you suspect, being a 5-letter sesame.
32d: the wordplay consists of the chemical symbols with U (for all to see) elided. That provides three of the four letters.
I have a full grid and think I know which two letters to change, but all I see for the confirming word is a 5-letter word running down which does seem apt, but Chambers does not have a 4,8 definition for it. Hint please