grammar - Verb volitional form (動詞の意志形) - usage

I have a couple of questions about the volitional form of verbs that I've become unclear on lately. Here is a Bible passage containing the grammar in question:

(Note that I'm using a Bible passage as I have before because it directly relates to the topic. I'm not trying to bring any spiritual discussions into this post.)

「主{しゅ}の名を呼び求める者はだれでも[救]{すく}われる」のです。ところで、信じたことのない[方]{かた}を、どうして呼び求められよう。聞いたことのない方を、どうして信じられよう。また[宣]{の}べ伝{つた}える人がなければ、どうして聞くことができよう。[遣]{つか}わされないで、どうして[宣]{の}べ[伝]{つた}えることができよう。　ー　ローマの[信徒]{しんと}への手紙 10[章]{しょう}:13-15[節]{せつ}

As the scripture says, "Everyone who calls out to the Lord for help will be saved." But how can they call to him for help if they have not believed? And how can they believe if they have not heard the message? And how can they hear if the message is not proclaimed? And how can the mesage be proclaimed if the messengers are not sent out? - Romans 10:13-15

So, my questions are:

1. 
   
   

   How does the volitional form work in this type of rhetorical question (without a か I might add)? Would the same meaning come across if these questions simply ended in 辞書形＋か (like どうして信じられるか)?

   
   


2. 
   

   Can Verb-意志形 always be replaced by Verb-辞書形＋でしょう／だろう in this type of question? Because substituting that into these questions seems to make a little more sense to me. どうして信じられるでしょう　→　"How can they (possibly) believe?"

   
   

   Of course, it does not make sense that you could substitute it when Verb-意志形 means "let's do X". (あとで外食しよう　→　"Let's go out to eat later", is clearly different than あとで外食するでしょう　→　"We'll probably go out to eat later")

   
   

Answer

Note that this isn't just the volitional, which for 呼び求める would be 呼び求めよう, it's the volitional of the potential form, and it's specifically paired with どうして. It is, as you say, a rhetorical question.

どうして呼び求めることができよう(か) would have the same meaning, I don't know why they use that form in some sentences and not in others in your quote.

This form has a stronger sense of "impossibility" than your other suggestions. From the speaker's point of view, it is impossible for someone to "call to him for help if they have not believed".

聞いたことのない方を、どうして信じられよう。 (聞かなければ、信じられない。)

Some commentary on どうして + ことができよう in Japanese: http://oshiete.goo.ne.jp/qa/5764430.html