What did Robert Frost mean in his April 7 1926 letter to B.F. Skinner?

Question

1. What does Frost mean by "Stevenson's in favor [...] consumptive"?
2. "Hardy's against God for the blunder of sex"?
3. "Sinclair Lewis's against small American towns"?
4. "Shakespeare's mixed"?
5. I don't understand the rest of this paragraph either, on page 521 of The Letters of Robert Frost, Volume 2.

      My attempt to get to the bottom of a fellow writer's stuff this time put this into my head: All that makes a writer is the ability to write strongly and directly from some unaccountable and almost invincible personal prejudice like [Robert Louis] Stevenson's in favor of all being happy as kings no matter if consumptive, or Hardy against God for the blunder of sex, or Sinclair Lewis’ against small American towns, or Shakespeare’s mixed, at once against and in favor of life itself. I take it that everybody has the prejudice and spends some time feeling for it to speak and write from. But most people end as they begin by acting out the prejudices of other people.

Answer 1

I don't know whether you were having trouble disentangling the sentence itself or only with guessing what Frost was referring to, but it's certainly a complicated sentence. Start with

All that makes a writer is the ability to write strongly and directly from some unaccountable and almost invincible personal prejudice.

Then (without so much as a comma to keep things tidy!) Frost lists examples of such "prejudices" that he thinks gave some famous writers their material:

Stevenson's [prejudice] in favor of all being happy as kings no matter if consumptive

Peter Shor in a comment identified this as a reference to a poem of Stevenson's, Happy Thought: The world is so full of a number of things, I'm sure we should all be as happy as kings, adding, "This actually seems rather unfair of Frost; it's a two-line-long child's poem and I don't think its sentiment necessarily reflects Stevenson's entire body of work."

Hardy's [prejudice] against God for the blunder of sex

Sex and the trouble it can lead to is indeed a pretty common theme in Thomas Hardy's writing, Tess of the D'Urbervilles is an obvious example.

Sinclair Lewis’ [prejudice] against small American towns

I haven't read any of Sinclair Lewis's works, presumably Frost thought that his work was mostly inspired by his dislike of small American towns!

Shakespeare’s mixed [prejudice] at once against and in favor of life itself.

Interesting statement, I don't know off-hand what he was referring to there.