Is に optional in this sentence?

Question

In this sentence:

わたしは、ほぼほぼ計画通りに過ごすことができた一年でした。

Does the に particle make "計画通り" an adverb?

Answer 1

Yes it's optional. The following sentences are both correct and feel exactly the same to me.

• ほぼほぼ計画通りに過ごすことができた一年でした。
• ほぼほぼ計画通り過ごすことができた一年でした。

に is not necessary in certain standalone adverbs, and it's optional in some adverbial phrases, too. ～通り is a phrase where に is optional. E.g., 予定通り(に)通過する, 予告通り(に)放送する, 練習した通り(に)やる.

You should remember when に is optional to answer a question like this.

See also:

• Difference between くらい and くらいに

Answer 2

Is に optional in this sentence?

No. It's necessary.

Does the に particle make "計画通り" an adverb?

Yes. The 「に」 particle has a lot of usages and makes a noun (or a noun-like phrase) adverbially connect to the verb in different ways.

In this case, No. 8 of this dictionary entry the most matches:

動作・作用の行われ方、その状態のあり方を表す。「直角に交わる」「会わずに帰る」
Indicates the manner in which an act is done, an effect is made or a state is being.

So 「計画通りに」 means something like "in a manner that is 計画通り (just as planned)."