Doug,

Hal Bush gave a very good presentation on Twain and Lincoln at the
Elmira conference last week.  You can also find some of Twain's thoughts
on Lincoln online at the following:

http://www.twainquotes.com/19070113.html

http://www.twainquotes.com/19010212.html

On Mrs. Lincoln at:

http://www.twainquotes.com/18680301t.html

As to Twain attending the Whitman speech in April 1887 -- I can
find no evidence he attended from searching the major daily
newspapers -- New York Times, Washington Post or Chicago Daily
Tribune. All three papers printed long lists of the famous who
did attend. These included some of Twain's colleagues--among them
William Dean Howells, (the Chicago Daily Tribune reported that
Howells was taking tickets at the entrance), Major Pond, John
Hay, Lawrence Hutton, Richard Watson Gilder, Moncure Conway, and
numerous others Twain knew.  No major daily paper that I have
access to in searchable format reported Twain in the list
of those who attended.

Barb