1) see what Silva Rhetoricae has to say about diasyrmus.
http://humanities.byu.edu/rhetoric/silva.htm

b) OED online, yes.

iii) it's hardly free; subscriptions for individuals are $295 per annum, or a low monthly rate of US$29.95. but you can get free access through proxy servers at many libraries. ask your local...

and finally, see how this tergiversating twit defines it:
http://home.mn.rr.com/wwftd/def.htm#diasyrm