When do you leave out the preposition in a relative clause?

A non-fiction titled "Do the Right Thing" published in 1998 has this sentence:

(1) Am I treating this stranger with the same consideration that I would a friend?

Another book (fiction) titled "Strong Rain Falling: A Caitlin Strong Novel (by Jon Land)" published in 2013 has this sentence:

(2) ...and treat the enemy with the same consideration with which they had treated the residents of Willow Creek.

Are both these grammatical and natural English?

If so, it seems to me that the that clause of (1), as well as the which clause of (2), is a relative clause. And that in (1) the preposition with has been left out of the relative clause.

Assuming that both are correct English, my question is why (1) is possible without the preposition with stranded at the end? (Edit: Is it because the relative clause of (1) lacks the lexical verb "treat"?)

Also, if preposition stranding is possible with these relative clauses, are these possible English?

(1a) Am I treating this stranger with the same consideration that I would treat a friend with?

(2a) ...and treat the enemy with the same consideration which they had treated the residents of Willow Creek with.

Edit: Here is another example without ellipsis, and it's taken from a hospital website in England:

(3) We Ask That Our Patients:

treat our staff with the same consideration that you would expect to be treated yourself. Violent, abusive or intimidating behaviour of any kind will not be tolerated and will lead to immediate removal from the practice list.

Now, should the editor of the website have added with at the end of the boldfaced portion?

Revamp: Since there has been some confusion due to ellipsis in (1), I would like to give this question a revamp.

After reviewing the answers and comments so far, I have concluded that the omission of "with," along with that of "treat," in (1) was simply due to ellipsis, and that the boldfaced clauses in (1), (2), (1a), (2a) and (3) are all relative clauses. (Any criticism would still be welcome as to any part of these conclusions.)

So let's forget about (1) and focus only on non-elliptical examples, i.e., (2), (1a), (2a) and (3). The issue now is whether, in each of these non-elliptical constructions, "with" is redundant, essential, or optional, on the meaning of which I will elaborate in the following three scenarios:

Scenario A: If it's redundant, (2), (1a) and (2a) will be ungrammatical, and only (3) will be grammatical.

Scenario B: If it's essential, (3) will be ungrammatical, and (2), (1a) and (2a) will be grammatical.

Scenario C: If it's optional, (2), (1a), (2a) and (3) will all be grammatical.

Which is the right scenario and why?

Edit: For those who support Scenario C, please elaborate on what it is in these examples that renders optional the use of preposition "with", when in general you are not to leave out a preposition from a relative clause if that preposition is part of the relative clause.

They are both grammatically correct.

With the same consideration in both cases is an adverbial prepositional phrase modifying the verb treat.

That I would a friend is an adjectival clause modifying consideration. The verb in the clause is implied, and in full form would be that I would treat a friend. While the preposition could be added along with the verb, it is not essential. If it were added, the possibilities are

Am I treating this stranger with the same consideration that I would treat a friend with?

Am I treating this stranger with the same consideration with which I would treat a friend?

While both are grammatically correct (assuming you have surrendered on the avoidance of terminal prepositions), they sound clumsy.

The originals sound fine.

• Am I treating this stranger with the same consideration that I would a friend?

I suspect that most readers here, like myself, and like the Original Poster, are likely to be red-herringed by the relative clause in this example. The deletion of the material in the relative clause has nothing to do with relative clauses at all - and this includes the deletion or otherwise of the preposition. It is not even due to the comparative structure used in the larger sentence.

The deletion of the verb and preposition here are just due to the ability of auxiliary verbs like would to take code. What is meant by this, is that material in the verb phrase following the auxiliary may be deleted if it is easily recoverable from the context. Here everything apart from the direct object, a friend, is easily recoverable because it has been mentioned in the previous clause. A friend, however, is a new topic in the conversation and must be repeated.

The fact that this repetition of material happens to be within the same sentence is coincidental, as is the fact that the deletion is taking place within a relative clause. We can observe would taking code in exactly the same way due to the same ideas having been covered in the previous sentence:

• I wouldn't treat John with respect. You, I would.

This seems to show that the deletion is more to do with would than with relative clauses - though I'm open to persuasion!

Consider a simpler example (1')

• I treat him with the same consideration that I would a friend

Several issues seem to be involved here, and a number of important distinctions need to be made.

First, and most important, this is an Equative construction and as such has its own special syntax.
Like Comparatives, than which no syntactic topic is more fraught, equatives are complicated.
So we should expect difficulties and confusion from the start.

Second, as noted, there has been deletion, by Conjunction Reduction, which is part of the equative.

Third, the subordinate clause is a Relative clause, which -- again -- has its own special syntax.
Every relative clause contains a reference to its antecedent noun phrase.
So where is the reference to consideration in the relative clause that I would a friend?
Following the usual triangulation, it's that. But what did that replace in the original clause?

• I would treat a friend with (X degree of) consideration

(the X degree of part comes with the equative construction, establishing a baseline standard)
Since consideration is an oblique noun phrase (i.e, not subject, not object, not indirect object),
it can only occur in a prepositional phrase, and treat governs the preposition with.

That produces the relative clauses

• the same consideration I would treat a friend with (Zero pronoun, stranded with)
• the same consideration that I would treat a friend with (that clause, stranded with)
• the same consideration which I would treat a friend with (which clause, stranded with)
• the same consideration with which I would treat a friend (which clause, pied-piped with)

but not

• *the same consideration that I would treat a friend (deleted with)
• *the same consideration with that I would treat a friend (that clause, pied-piped with)

because (a) treat requires with, and (b) Pied-Piping only applies to wh-clauses, not that-clauses.

Next question, what happens when one deletes treat in the relative clause?
Answer: we still expect it, under the terms of conjunction reduction, so it still requires with.

• *the same consideration that I would a friend (deleted treat, deleted with)

There's a lot more one could talk about here, like the restrictions on relative clauses of manner,
but this is long enough already.

Executive Summary: The more constructions a sentence contains, the more syntax it has.