Why does 'than which' sound incorrect as a preposition + relative pronoun?

Question

I cannot, but please tell me if you can, diagnose why than WHICH below still sounds incorrect and strange. Did I err in my rewrite of 3 below as two separate Independent Clauses (to diagnose my problem, per this answer)? Please explain all steps and thought processes.

_{I commence the numbering at 3, as the Accepted Answer below already uses the integers 1, 2.
Source : The Adventures of Ferdinand Count Fathom, Volume 01 by Tobias Smollett (1721-1771)}

[3.]  [...]  his letters  [...]  lay him under the necessity of soliciting preferment in the army, than WHICH nothing was farther from his inclination.

⟸ 4. His letters lay him under the necessity of soliciting preferment in the army,
nothing than WHICH was farther from his inclination.

⟸ 5. [His letters lay him under the necessity of soliciting preferment in the army] +
[Nothing than [being] under the necessity ... in the army was farther from his inclination.]

Answer 1

Introduction

Your quotes are ancient or dated texts: Anselm is from the 12th century; and the above is from the 18th century. Having said that, I believe that we can understand them in the same way that we understand 'pied-piping' in general.

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Background

Let's consider a couple of simpler examples. I'm going to use of which instead of than which to make it look and sound more familiar.

[1i] It was a war. ​　　　 [1ii] No word of this war was allowed to leak to the outside world.

We can combine [1i] and [1ii] by pied-piping "of this war", resulting in [1iii]:

[1iii] It was a war, of which no word was allowed to leak to the outside world.

Let's try another example:

[2i] It was a secret. ​　　　 [2ii] No one knows of this secret.

We can combine [2i] and [2ii] by pied-piping "of this secret", resulting in [2iii]:

[2iii] It was a secret, of which no one knows.

Note that the two examples are pied-piped in a similar manner. The difference is that in [1] the pied-pied part is from the subject of [1ii], but in [2] the pied-piped part is from the predicate of [2ii].

This allows us to understand why [1iii] and [2iii] can be rewritten as [1iv] and [2iv] respectively (note that [1iv] is identical to [1ii] and [2iv] is identical to [2ii]):

[1iii] It was a war, of which no word was allowed to leak to the outside world.
= [1iv] No word of this war was allowed to leak to the outside world.

[2iii] It was a secret, of which no one knows.
​= [2iv] No one knows of this secret. (NOT: No one of this secret knows.)

As you can see, the steps you proposed are not quite relevant.
Relating of which to the wrong part of a sentence will get you an incorrect meaning.

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Our example

By way of analogy, the sentence:

He suppressed all his letters of recommendation, which he justly concluded would subject him to a tedious course of attendance upon the great, and lay him under the necessity of soliciting preferment in the army, than which nothing was farther from his inclination;

or your ellipted version of it (ie your 3 above):

His letters would lay him under the necessity of soliciting preferment in the army, than which nothing was farther from his inclination.

should be read as:

Nothing was farther from his inclination than (being) under the necessity of soliciting preferment in the army. (And his letters would lay him so.)

or as an alternate reading:

His letters would lay him under the necessity of soliciting preferment in the army.
Nothing was farther from his inclination than such condition.

Our example is similar to [2iv], not [1iv].

Hope this helps!

Answer 2

It is easier than it seems.

He had overdone it the night before, and nothing was farther from his desire at that moment than a martini.

When he was offered a martini, than which nothing was farther from his desire at that moment, he declined.

P.S. Contemporary American English would not use a "than which" clause; it would go something like this:

When he was offered a martini, which a glass of spring water seemed much better than, he declined.