"These shoes are too small!"
I think saying without 了 does not necessarily make the sentence more harsh and direct. That could be an illusion of the effect of saying the sentence with a raised voice and stressed tone.
Sentence like this is emotionally quite neutral:
That book is too high, he can't reach it
(lit. The book has been put too high, he cannot take it.)
We use 太... 了 to wrap around a phrase to express an exclamation. The emotion can vary: anger, despair, disappointment, pity, etc.
e.g. 你太不听话了！/ You are too disobedient!
太可怕了！/ It is too frightening!
她太可怜了！/ She is too miserable!
The other day we discussed about ...太不好了！This is acceptable as an exclamation, but would otherwise become grammatically incorrect without 了. It is not allowed to say 太不好, as it is identical to saying "too not good" in English and is unacceptable.
So 了 does not always go with 太, but if you are making an exclamation with 太... 了, do not forget about it.
No, the "too" is translating "太". You can leave out the "le", but it doesn't sound as natural and sounds a bit harsh. We usually use "le" in the end in this case to soften it up. For example:
这本书太厚了！/ 这书太厚！ 这车太旧了！/ 这车太旧！ 今天太热了！/ 今天太热！ All these are correct but the latter ones sound more harsh/stronger.
Yes, to make the sentence sound natural