I didn't want to answer this because I found the answer by searching the web for the three authors' names, restricting the domain name to that of a popular online dictionary. I also had some trouble relating some of the clues to the answer. But nobody else has answered correctly, so here it is.
The answer is
I'm an old-fashioned word that is rarely used
The word seems to have been most popular between the early 19th and early 20th centuries.
An example of Latin and English fused
Not sure about that one. I suppose that it is because some dictionary etymologies give the source as Latin, but the word didn't appear until late Middle English, so it's perhaps just as likely to have been formed directly from English opinion.
Charles Lever used me, so did Dickens
and Sir Arthur Conan Doyle - wow, the plot thickens!
A pretty large pile of building, I opine, and a pretty long job! The Uncommercial Traveller
The reason is, I opine, that each doth wait for his neighbour to make a move. Micah Clarke
"I opine that the granddaughter should be got rid of," said the Colonel. Sir Brook Fossbrooke, Volume I.
- Also Charles James Lever:
I opine that the same judgment might be passed upon a great many? A Day's Ride
You may be forgiven for mistaking me
for an ode to a German Christmas tree
By replacing "pine" with "Tannenbaum," opine becomes O Tannenbaum
If you think you know the answer, have a go
It's what you think, not what you know!
Opine means to hold an opinion, so it refers to what one thinks, not what one knows.
As to the hint,
I can only understand that it is intended to point away from the direction of words that resemble "Tannenbaum," but I do not understand the meaning of "it's the ode you could mistake it for."