“As shown” or “As is shown”?
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When we refer to figures in a paper, we should use as shown instead of as is shown. Is in this case is superfluous. Similar usages include as discussed..., as pointed out... etc.

References
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https://ell.stackexchange.com/questions/102045/may-i-drop-is-in-constructions-like-as-is-shown-by-the-example-of

“unless otherwise stated” or “unless stated otherwise”?
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Both the two forms are correct and mean the same thing. From the Goolge ngram website, it seems that unless otherwise stated is much more popular than unless stated otherwise.

References
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https://english.stackexchange.com/questions/37033/unless-stated-otherwise-or-unless-otherwise-stated

When to use comma before a conjunction (“and”, “or”, “but”)?
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There are three typical uses of conjunctions:

To connect two independent clause. A comma should always be used before conjunctions.

The rain stopped, and the sun came out.
I like to watch movies, but he likes to read novels.

To connect two dependent clause. A comma should not be used.

He is a healthy and happy boy.

To connect three or more items or clauses. A comma is optional.

I have travelled to China, Japan, and Thailand in Asia.
Should I go to swimming, play basketball, or watch TV?

References
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https://english.stackexchange.com/questions/30516/should-i-use-a-comma-before-and-or-or

“if” or “whether”?
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If is used to express a condition, while whether is used to give two choices. For example:

I do not know if the exam will take place in Saturday or Sundary.

I do not know whether the exam will take place in Saturday or Sunday.

The two sentences above have subtle differences. The first sentence means that the exam may take place in Saturday or Sunday, but it may also take place in other date. The second sentence means that the exam will take place either in Saturday or Sunday. There is only two choices.

You should also note that whether should be used after a preposition. For example:

We should depend our decision on whether he will come or not.

References
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https://www.youtube.com/watch?v=Klnroe1UBRs

“cannot” or “can not”
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According to discussion here, cannot is a modern way of writing can not. Technically, there is no difference between them. In practice, cannot is used far more frequently than can not.