grammar - understanding 対する

これが私の手紙に対する彼の返事だ。 This is his answer to my letter.

自分の中の彼に対する気持ちをどう表現していいのか分からないの。I don’t know how to express how I really feel to him.

彼に対して、彼を好きという私の気持ちを、どう表せばいいか、わからない。 I don't know how to express to him that I like him.

Often when I run into 対する in colloquial sentences like these, its meaning appears somewhat extraneous. It seems to emphasize the direction of the action in an obvious way. I am tempted (mistakenly?) to think that in these kind of sentences, the meaning expressed could be sufficiently communicated with just に. Is 対する just adding extra emphasis, or I am I misunderstanding some of its meaning or grammatical purpose? How does it add to what is being communicated? Does it add a measure of gravity to the statement?

For instance, if I was to remove the 対する, would these sentences become grammatically incorrect, or would the meaning drastically change? Perhaps they just start to sound unnatural or poorly constructed...

If you see any mistakes in translation feel free to point them out. Thank you.

これが私の手紙に彼の返事だ。

自分の中の彼に気持ちをどう表現していいのか分からないの。

彼に、彼を好きという私の気持ちを、どう表せばいいか、わからない。

Answer

As Tsuyoshi Ito answers, when you modify a noun, you can put a genitive (の)-marked noun phrase, or a relative clause. Notice that 対する retains the morphological form of a verb (although it does not fully inflect as with real verbs). Thus, at least etymologically, you can consider the 対する-phrase as a relative clause.

[(それが)手紙に 対する (ところの)]返事

'a response [such that it aims against the letter]'
'a response that is aimed against the letter'  

Here, the に phrase is an argument of the verb 対する. You cannot omit the verb and retain its argument. The (それが)手紙に in the following will be a strange syntactic object; it has an argument but not the predicate, and is still intended to function as a relative clause. That is why it is ungrammatical.

* [ (それが)手紙に ](ところの)返事

If you want to say it short, you can use へ in front of the genitive case like this:

? 手紙 への 彼の返事

自分の中の彼 への 気持ち

although the first example is awkward because へ can be used only against things that will be affected by the action.

In your last example, the に-phrase simply functions as an argument.

彼 に 気持ちをどう表すか