How does Acetaldehyde accelerate mitosis?
First of all Alcohol and Acetaldehyde are both cytotoxic. Along in our development we learned to deal with both substances, which is shown by specific metabolism pathways to break them down (for example alcohol dehydrogenase, acetaldehyde dehydrogenase, Cytochrom P450 and so on).

Alcohol and Acetaldehyde both have a negative effect on mitosis (which explains a good part of the negative effect which alcohol has on developing embryos).
Examples can be the following articles:


Acetaldehyde causes a prolongation of the doubling time and an
increase in the modal volume of cells in culture.
Alcohol inhibits cell mitosis in G2-M phase in cell cycle in a human
lymphocytes in vitro study.
Ethanol induces transient arrest of cell division (G2 + M block)
followed by G0/G1 block: dose effects of short- and longer-term
ethanol exposure on cell cycle and cell functions.
Ethanol induction of sister chromatid exchanges in human cells in
vitro.


Alcohol also has some effect on cancer - probably not by directly causing it, but at least as an additional risk factor. One article I found ("Alcohol and Cancer") mentions the following possibilities:


  The mechanisms by which alcohol consumption exerts its carcinogenic
  effect have not been defined fully, although plausible events include:
  a genotoxic effect of acetaldehyde, the main metabolite of ethanol;
  increased oestrogen concentration, which is important for breast
  carcinogenesis; a role as solvent for tobacco carcinogens; production
  of reactive oxygen species and nitrogen species; and changes in folate
  metabolism.


Using Pubmed you can find more articles on that topic.