If I have the を replaced with はin both sentences,could they be alright? I suppose I can't have two は's in the first one. So I might have it done as "これが新幹線は大阪までです". The other one would be "歩くのは二十分くらいです".
No for #1. これが新幹線は大阪までです doesn't make sense, either. It should be これは大阪まで行く/大阪行きの新幹線です for "This is the shinkansen to Osaka."
この新幹線は大阪までです is understandable, but この新幹線は大阪行きです is more common. These expressions are equivalent to "This shinkansen is to Osaka".
As for #2, 歩くのは二十分くらいです is understandable, but this sentence means "walking is about twenty minutes", so it's more likely used in a context where you walk partially when moving to somewhere.
e.g.
公園までは3時間くらいかかりますが、ほとんどはバス移動で、歩くのは二十分くらいです。
歩いて二十分くらいです is more common for explaining the distance to a place.
e.g.
寮から大学までは歩いて二十分くらいです。
In the same dictionary that stated that こわがる is an intransitive verb. All the examples therewith have the verb go with を though.
I saw the following excerpt at
自動詞・他動詞 - ことば・辞書・日本語文法(2).
こわがる【怖がる】(自五)
怖いと思う（様子をする）。 [
新明解国語辞典第七版]
こわ‐が・る【怖がる（▽恐がる）】他五
怖いという気持ちを態度に表す。びくびくする。おそろしがる。
When I asked around,someone gave me this link. It seems to say こわがる can be intransitive or transitive depending on the context. Certainly I could well be wrong in fully understanding the Japanese text.
It's not "こわがる can be intransitive or transitive depending on the context". As the writer of the site says 心理的な動詞で、ヲ格をとる場合、新明解は自動詞とし、明鏡は他動詞とする傾向がある, the definition differs depending on the dictionary. He also says 単純に言って、ヲ格をとる例を挙げながら、自動詞だとする新明解の立場は、何らかの説明を要するように思います at the end of the page, i.e., 新明解国語辞典 should explain why they think those verbs are intransitive despite being used with を.
Here's a related post I wrote before.
A dictionary of basic japanese grammar? Maybe this one? A Dictionary of Basic Japanese Grammar: Seiichi Makino, Michio Tsutsui: 8601405255785: Amazon.com: Books
As I mentioned there, the problem is that the definition of 自動詞/他動詞 is not clearly stated in Japanese grammar. There are some hypotheses. The following page explains about the three major definitions concisely.
(all in Japanese)
I believe the most useful and easy-to-understand one for non-native learners, and therefore the most commonly used in textbooks, is "whether the verb can take を or not", so I've been using this definition in this forum. Of course you can have questions about the definition of transitive and intransitive verbs, but it's not so useful to think about it too much, especially in the early stage of learning, I think. It's more helpful to remember that 怖がる takes を even though some dictionaries call the verb intransitive, I believe.
You would come across similar cases, for instance "noun vs. adjective" in
this thread. As I pointed out there, the definitions in dictionaries could sometimes be useless, or could even confuse learners. The point is how the word is really used, not what the definition of the word is in dictionaries. My two cents.