val. in Klingon is a complete sentence, so it's best to translate it into English with a complete sentence: She is smart.
val can also act as an adjective, when it is behind a noun -- for example, be' val "a smart woman" or "the smart woman".
Klingon doesn't really have infinitives -- the sorts of things that would use infinitives in many European languages get treated differently in Klingon.
But the dictionary form is the bare verb stem, which also happens to be the form for "he/she/it/they ...", since the verbal prefix for that is nothing.
In other words, from val "smart" you add the prefix jI- to get jIval "I am smart", but you add nothing to get val "he is smart, she is smart, they are smart".
No, it is not possible. In English, "smart" is an adjective and so you need a verb like "is" to connect it to a noun in a sentence like, "Mara is smart."
In Klingon, val is a verb and so you can connect it directly to a noun in a sentence like val mara. You might want to think of it as already having the "be" in the definition: val="be smart".
Since 'oH is not being used as a verb in the sentence you suggested, it can only be a pronoun. You can't use both a pronoun and a noun in the subject position. Thus your suggested sentence is ungrammatical.
All of the pronouns can be used like the verb "to be". 'oH normally just means "it" (referring to something that is either not a being or not capable of language, or both), but when used as the verb "to be" it means, "it is". For a being which is capable of language, we instead use ghaH to mean either "he/she" or "he is/she is". As another example, we can use maH to mean "we/us" or "we are". As in tlhIngan maH.