"That" as a relative pronoun and "in + verbal noun"
Below is a sentence from a book review of The Economist. I am a little unsure of my comprehension of "that" and "understanding" in the sentence.

1.
The book is likely to be the closest [that anyone will come to understanding the thinking behind a foreign policy that has many critics.]

I assumed "that" here would be the relative pronoun modifying "the closest (book)." And I supposed the verbal noun "understanding" here should be understood as "in understanding" like in the sentence "In planning public expenditure it is better to be prudent."

Therefore I interpreted the sentence as "The book is likely to be the closest (book) to which anyone will come in understanding(= when he tries to understand) the thinking behind a foreign policy that has many critics."

Q1. Are my interpretations of "that" and "understanding" correct?
Q2. If it is right to interpret "understanding" here as the equal with "in understanding", is it possible to omit "in" in a similar case?Thank you for reading to this end. Perhaps you would explain

Question

"That" as a relative pronoun and "in + verbal noun"
Below is a sentence from a book review of The Economist. I am a little unsure of my comprehension of "that" and "understanding"  in the sentence.

1.
The book is likely to be the closest [that anyone will come to understanding the thinking behind a foreign policy that has many critics.]

I assumed "that" here would be the relative pronoun modifying "the closest (book)." And I supposed the verbal noun "understanding" here should be understood as "in understanding" like in the sentence "In planning public expenditure it is better to be prudent."

Therefore I interpreted the sentence as "The book is likely to be the closest (book) to which anyone will come in understanding(= when he tries to understand) the thinking behind a foreign policy that has many critics."

Q1. Are my interpretations of "that" and "understanding" correct?
Q2. If it is right to interpret "understanding" here as the equal with "in understanding", is it possible to omit "in" in a similar case?Thank you for reading to this end. Perhaps you would explain

Answer

Hello!
Q1: You understand the use of 'that' here correctly. 'Understanding' means more 'to be able to understand' here, which is what you have correctly understood even if your substitution of "in understanding" doesn't make much sense to me in this context.

The book is likely to be the closest (book) to which anyone will come to being able to understand the thinking behind a foreign policy that has many critics

Q2: In planning... means 'while planning' or 'during planning'.  So, no, you couldn't take out the 'in', but you could rephrase it. When planning public expenditure..., or While planning...

Answer

In your sentence, “that” is actually used to give more information about the adjective “closest”. It isn’t really used as a relative pronoun; it is used to introduce a noun clause acting as an adjective complement.

“that anyone will come to understanding” is a “noun clause” giving more information about the adjective “closest”.

It is the same structure if I say: 
I am afraid that anyone will come to understanding my answer.
“Afraid” is an adjective, and the “that” clause gives more information about the adjective “afraid.”
“That” is optional in this case. 
I’m afraid anyone will come to understanding my answer.” is correct, as well.

If you write the sentence until “closest” and put a full stop after it, the sentence still makes sense. It has less details, but it is still a complete and correct sentence.

“Understanding” here, is used as a gerund because “come to” is a phrasal verb which has many meanings, but in this case, it can be understood as “to get” the understanding. 
This is similar to “look forward to” + gerund.

Hope that makes sense.