yay conditionals!! i love conditionals!!! (i'll try to keep it brief though haha)
let's do なら first because this is the one that i personally feel is not made very clear by textbooks and such.
sometimes なら is called "assertive" なら, but personally i don't feel like that's a very helpful qualifier...for me, i like to think of it as a conditional based on assumptions. the gist is often something like, "assuming what you say is true/will happen..."
so, we can see now why なら doesn't exactly fit with the example from your textbook:
その話が本当なら、うれしいです。 = if that story is true (as you say it is/as it seems to be), then i'm glad.
i don't think this sentence is grammatically incorrect or unavailable; however, it means something different than what i believe your textbook is going for. so let's go to 〜としたら to see the other option.
〜としたら = "if it were the case..."
there's waaaaay more to be said on the 〜たら form than i'm going to write in this post (rest assured i've written about it extensively elsewhere LOL), but i'll say that anything to do with 〜たら has the potential to be "unreal," meaning you can use this form to talk about things that haven't actually happened.
the phrase 〜とする means kind of like "to assume" (in this case), so in that sense it is similar to なら. however, the assumption that comes from 〜とする is often an imaginary one:
Xなら = if X is in fact true (as the evidence suggests)
Xとする = imagining for a moment that X were true...
see the difference? 〜とする has the ability to deal in "unreal"-ness, which なら doesn't.
so, if we tack a conditional ending onto 〜とする, we get this kind of sentence:
その話が本当だとしたら、うれしいです。 = if that story were true (assuming for a moment that it is, though really i don't know), i would be glad.
hopefully the nuance comes out with my translation here. using 〜としたら puts the conditional squarely in the realm of unrealness, i.e., things that can't be confirmed (at the time of speaking) to be true or false.
so, probably at this point we can make a judgment on op's practice sentence. our goal here (i believe) is to get a sentence that means, "it sounds like emi got dumped. if that's true, she was probably crying all day." here's the original that was proposed:
ねえ、えみちゃんはフラれたそうだ。それ❓だとしたら❓、おそらく一日中泣き通していた。 = it sounds like emi got dumped. ❓if that were true❓, she would probably have been crying all day.
what do you guys think? for my part, i think なら is our better option here! that's because of the first sentence: since there's a 〜そうだ ("i heard") clause, we have evidence that the situation is actually true. that means we don't have to relegate ourselves to unrealness or imaginging that the situation might be true.
ねえ、えみちゃんはフラれたらしい。それなら、おそらく一日中泣き通していた。 = it sounds like emi got dumped. if that's true (which it seems to be), she was probably crying all day.
by using なら, we can show we're relying on external evidence to make this if-then claim.
on the other hand, we might find 〜としたら in a situation more like this:
えみちゃんは最近元気なさそうだよね。フラれたとしたら、こんなうっとうしい感じでもするでしょ？ = emi has seemed kind of down lately. maybe if she had been broken up with, she would be feeling as depressed as this?
in a case like the above, we don't have much good evidence to suggest emi has actually been dumped; rather, we're speculating on the cause of her bad mood. so, we can use 〜としたら to talk about that unreal situation, i.e., a situation that can't be confirmed.
phew, that was a lot! i hope it was helpful, and as always anyone can feel free to chat with me/ask me questions about grammar whenever :D
