In my opinion (<--!!) it is the direct contact that is the MAIN problem.
My reasoning is that if it is ingested the resulting concentration in the blood is really quite low. This would also go along with the mouthwash story, since you (normally) do not drink that.
In case of alcohol and smoking the effect is locally synergistic, the idea is that ethanol acts as a cosolvent and facilitates the entry of other carcinogens into surface cells.
I cannot HELP but think that the effect of alcohol would have to depend on what it is (i.e. pure ethanol or cognac vs, say wine). So it should depend on the concentration and also on the duration of the contact. I am however not aware of any studies that address this.

What has been floating around is this:
Enhanced penetration of nitrosonornicotine across oral mucosa in the presence of ethanol. Journal of Oral Pathology & Medicine
Volume 15 Issue 5, Pages 276 - 279 28 Apr 2006 DOI: 10.1111/j.1600-0714.1986.tb00623.x
** note that is an old paper 1986

And then there is the concept that the carcinogen (acetaldehyde) are created locally from ethanol by different means.
(Acetaldehyde is produced from ethanol by oxidation and can react with proteins and DNA).

Acetaldehyde production from ethanol by oral streptococci Oral Oncology, Volume 43, Issue 2, Pages 181-186 (2006)
** Here produced by bacteria.

Demonstration of ethanol-induced protein adducts in oral leukoplakia (pre-cancer) and cancer. J Oral Pathol Med. 2008 Mar;37:157-65.
** they looked at acetaldehyde protein adducts and found that in leukoplakia and SCC of excessive alcohol users you find protein adducts. This is interesting but I did not see (nor did I look past the abstract!) that there was a control where the looked at leukoplakia and cancer protein adducts of NON alcohol users. If the latter had less protein damage that would be even more convincing. Cells that are in a precancer and cancer state are quite unbalanced (and may just damage proteins on their own) without any outside help.

Even if things seem clear it can get murky.
For example if you take antabuse this would increase the level of acetaldehyde markedly when ethanol is consumed (=instant hangover).... yet the same drug is now investigated for cancer treatment (different mechanism though).


soo? I do not smoke and I drink wine occasionally and then I also drink water (rinse). And just to muddy the waters some more, acetaldehyde also occurs in fruit and coffee etc.

Cheers

M