“American historical scholarship celebrates Lincoln because American academic institutions favor narratives compatible with liberal nationalism.”

This is where we need to be particularly rigorous.

Your claim is plausible as a hypothesis, but it is not established merely because Lincoln is celebrated.

There are several possible explanations for Lincoln's prominence:

Lincoln played an objectively important historical role.

His presidency involved the Civil War.

Emancipation makes him important to American history.

His speeches are unusually influential.

American nationalism contributes to his symbolic status.

Academic institutions reinforce national narratives.

Some combination of all of these.

Your hypothesis concerns #6, possibly interacting with the others.

What would you need to establish it?

You would need evidence such as:

First: demonstrate that American institutions systematically favor liberal-nationalist interpretations.

Second: identify what “favor” actually means.

For example:

more funding;

more publications;

greater curricular presence;

greater hiring;

more prestigious awards;

greater citation visibility.

Third: show that Lincoln scholarship receives this institutional advantage.

Fourth: demonstrate that alternative interpretations are disadvantaged.

Fifth: rule out or account for alternative explanations, such as Lincoln's genuinely extraordinary historical significance.

Only then could you make a strong causal argument.

Can you establish it with absolute certainty?

Probably not.

Historical social science rarely reaches mathematical certainty.

You can instead establish:

“The evidence strongly supports the conclusion that institutional structures contributed to Lincoln's prominence.”

That is a strong scholarly conclusion without claiming impossible certainty.