Hi Sue,
> How well did Clemens speak/write German?

That depends on how you define "well." -- I'm assuming you have a
specific reason for asking this question. It might be easier to respond to
your research interest if you would specify the context for your question.

Generally,  a fair amount of scholarship on the topic is already available,
for example:

Cracroft, Richard H. "'Exactly the German Way': Mark Twain's Comic
Strategies with 'The Awful German Language'." Thalia: Studies in Literary
Humor 13, no. 1-2 (1993): 11-21.

Hemminghaus, Edgar. "Mark Twain's German Provenience." Modern Language
Quarterly: A Journal of Literary History VI, no. December (1945): 459-478.

-----. Mark Twain in Germany. New York: Columbia University Press, 1939.

Kersten, Holger. "'Human Natur in a Forren Aspeck': Mark Twain's Encounters
with German Culture." Mark Twain Review (The Mark Twain Circle of Korea) 4
(1999): 47-72.

-----. "Mark Twain and the Funny Magic of the German Language." New
Directions in American Humor., ed. David E. E. Sloane Tuscaloosa, Ala.:
University of Alabama Press, 1998. 284-305.

Klett, Ada M. "'Meisterschaft' or the True State of Mark Twain's German."
American-German Review VII (1940): 10-11.

Krumpelmann, John T. Mark Twain and the German Language. Baton Rouge:
Louisiana State University Press, 1953.

Schultz, John Richie. "New Letters of Mark Twain." American Literature: A
Journal of Literary History, Criticism, VIII (1936): 47-51.

Sewell, David R. Mark Twain's Languages: Discourse, Dialogue, and Linguistic
Variety. Berkeley and Los Angeles: University of California Press, 1987.

Thomas, Ursula. "Mark Twain's German Language Learning Experiences."
Teaching German in America: A Prolegomena to a History. Eds. David P.
Benseler, Walter F. W. Lohnes and Valters Nollendorfs. Madison: University
of Wisconsin Press, 1988. 133-143.

Wecter, Dixon. "Mark Twain As a Translator From the German." American
Literature: A Journal of Literary History, Criticism, and Bibliography XIII

(1941-1942): 257-263.

Weishert, John J. "Once Again: Mark Twain and German." Mark Twain Journal
XII, no. Summer (1965): 16.

Best,

Holger