Your family tale does not match the facts of history.
Ellis Is records indicate LACANSKY, from Croatia settling in Peoria and LASANCZ from Hungary settling in Madison, IL (next to St Louis MO)
The Archduke was shot by Gavrilo Princip, a member of the Serbian Blackhand (Crna Ruka) Society, which had strong ties to the Serbian Monarchy. There was no denounciation nor renounciation of the Habsburg Monarchy at any time. Some members of that family still exist and are somewhat prominent in Europe.
There is an ancestral file for Elmer Homer Joseph LACHANCE found at Family Search..
You will find a Soc Security Death record for Elmer LACHANCE... you may purchase his application which will have his parents names and his birthplace listed. It is always best to make note of family records and tales.. but it is best to start from you and work backwards... it appears that if the SS Record is your Elmer that would be the place to begin. You can locate the record and the SS number and send for the application.http://www.familysearch.org/Eng/Search/frameset_search.asp
Just a side note LACHANCE is an old French family name in that region, according the link belowhttp://www.rootsweb.com/~moomahs/surnames.htm
Also LACHANCE is found in the cemetery you mentioned, showing Elmer born 1913 but there are other LACHANCE with birthdates in the 1890shttp://www.rootsweb.com/~moomahs/surnames.htm
Also you may wish to post a message at the Washington County gen page or look at all the info on LACHANCE in that county dating back to the 1800shttp://www.rootsweb.com/~mowashin/
And... there is even a message forum dedicated to LACHANCE surnamehttp://genforum.genealogy.com/lachance/
And if you search for "LACHANCE genealogy" at Google.com you will find over 3,100 hitshttp://www.google.com