Smoke if you've got 'em

On a plane from Atlanta to Asheville last year, I got almost half of Convert by Mr. Lemon out - I saw that there was a Harry Potter theme from identifying "Diagon Alley" in the extra letters, and I started to pick up more of the tricks that help with solving Listeners. I never did finish it, but I was looking forward to seeing what Mr. Lemon had in store with Tobacco.

16 answers need metallic changes, remaining clues lead to an answer with an extra letter. A message makes it all clear.

Starting on the across clues, nothing jumped out until 12 - (A1,TITIAN)*,ED makes INTITATED with an extra A, and right after it is DIAL + C (hidden). 18 looks like the first answer that needs changing - LO in SAL giving SALON, and a letter needs to be removed (since I'm writing this well after coming up with that, I'm pretty sure SALON isn't right).

Then a bunch of unsolveables, until I hit 21 across... (OIKS)*,MET gives KISMET + O, S(CAN)S, then (LAIC,TYPE)* gives ECTYPAL + I, didn't know 27, (t)RAIL+S, 32 looks like something...ISHLY, a peek at Bradfords and there's SNOTTY for midshipman, so SOTTISHLY + N, and 33 is a clue I like a lot - TOO in OLEO (appearing in 73% of New York Times crosswords) gives O'TOOLE + O. I have almost all the bottom half of acrosses written in.

Downs... trickier. 4 is SAGINATE and needs changing. 6 is SCREE,N, another change (change appears to be losing one letter, or some modification that leaves a word one shorter). For 7, (ANY,SIOUX)* about AL gives ANXIOUSLY + A, 8 is SE,RA+N, and then a bit of a break.

16 is (KAREN) in SOOT which gives SNAKEROOT, and since I have the K,E,R and O, then the modification looks like it's in the first three characters. Sn and Na are chemical symbols for metals - are we merging the two and getting rid of the N?

20 looks like (LINT)* in GAGS to give GATLING+S (that fits). If 23 is C(LOTH)O+N (fits), 26 is PAOLO+I, and a trip to Bradfords to look up cattle gives GYAL so that's an extra N to finish off.

With the words I've written in, I can see a few more clues - 1 ac is BAR,BAR,Y COAST+T (nice clue). I scanned this a little early, I saw 2 is A,RUN,HEM to give ARNHEM+U. Word Wizards suggests TELEOLOGICAL will fit at 35, it has an anagram of COLLEGE and ALI, but I don't see where the rest of the clue could come from.

Since BARBARY COAST is in place, making 4D start with A, there goes the merging metals idea - are we just losing the S so the word starts with a metal symbol (Ag)?

The extra letters look like this T?BAC?OI?SNO?U?AN?DS?N?I?N. TOBACCO IS NOT(NOW?) U AND S?N?I?N? I've got nothing.

And I am stuck...

Looking at it now, I think it probably is removing the first letter so that answers start with the symbol for a metal, that would explain the metallic change, and although it doesn't match what I have in the grid, my need to change answers can all do this...

SALON - ALON (but that wouldn't work with SAGINATE??)
SAGOUIN - AG
SCANS - CANS
35 may have been STELEOLOGICAL to becomne TELEOLOGICAL?
SAGINATE - AG
SCREEN - CREEN

Dammit - is it S that needs to be taken off?
SNAKEROOT - NAKEROOT

Was I this close?

Well done Mr. Lemon - victory to the Listener Crossword. Listener 17, George 20. Current streak, Listener 1.