The 83rd Illinois came from the Galesburg area which includes Knox College. As you know,a group of students enlisted and formed a full company - Virgil Earp was in Co. C.
There were nine Earps in Illinois regiments - two in the 83rd - Virgil and Francis. As I stated, Wyatt was too young to serve.
As to if he was at Dover in February, 1863, email Ft. Donelson NBF historian Jim Jobe (tell him I sent you) at: Jim_Jobe@nps.gov and see what he says about that. I honestly don't know but would assume that he was there.
The Lincoln Library and Museum in Springfield, IL has a numebr of manuscript collections for the 83rd Illinois - a few of which I have been through personally.
Knox College has 83rd Illinois stuff too and their website mentions the students that joined up - so they would be a good soruce for you as well.