My Japanese dictionary says that はるばる is an adverb and sometimes is used with と. So you are right — the existence of と doesn’t change the grammatical role of はるばる. In fact, はるばる and はるばると are pretty much the same in terms of both usage and meaning.

As background, sometimes adverbs in Japanese are followed by と or に. (See https://kotobank.jp/word/副詞-123983; the 語形的分類 section in ニッポニカ explains this.)

For these words, adverb+と(に) is the wrong separation. “adverb+と” is one adverb. Whether you add と or not depends on each person.

The following is just my opinion as a native speaker of Japanese. This is totally not a linguistic explanation and my major is not linguistics.

Ending sentences with an adverb without と or に seems unusual (though of course there are exceptions, but I think most of them are in the narration of TV shows or literature).

So without a と or に following the adverb at the end of the sentence, it makes me feel like the sentence has ended suddenly. Adding と at the end of the sentence tells readers that there is something omitted at the end of the sentence, for example “やってきた”.

The と helps me recognize that this sentence includes an omitted phrase at the end, and as a result I don’t feel that it ends suddenly, making it feel more natural (especially in a language like Japanese where omitting things is common).

My Japanese dictionary says that はるばる is an adverb and sometimes is used with と. So you are right — the existence of と doesn’t change the grammatical role of はるばる. In fact, はるばる and はるばると are pretty much the same in terms of both usage and meaning.

As background, sometimes adverbs in Japanese are followed by と or に. (See https://kotobank.jp/word/副詞-123983; the 語形的分類 section in ニッポニカ explains this.)

Whether you add と or not depends on each person.

The following is just my opinion as a native speaker of Japanese. This is totally not a linguistic explanation and my major is not linguistics.

Ending sentences with an adverb without と or に seems unusual (though of course there are exceptions, but I think most of them are in the narration of TV shows or literature).

So without a と or に following the adverb at the end of the sentence, it makes me feel like the sentence has ended suddenly. Adding と at the end of the sentence tells readers that there is something omitted at the end of the sentence, for example “やってきた”.

The と helps me recognize that this sentence includes an omitted phrase at the end, and as a result I don’t feel that it ends suddenly, making it feel more natural (especially in a language like Japanese where omitting things is common).

My Japanese dictionary says that はるばる is adverb and sometime used with と. So you are right. existence of と doesn’t change the grammatical role of はるばる. はるばる&はるばると is almost same usage and meaning. Sometimes Japanese adverb come with と or に.  (https://kotobank.jp/word/副詞-123983 語形的分類 section in ニッポニカ explains this) In these cases, adverb+と(に) is wrong separation. “adverb+と” is one adverb. “Adding と or not “ is depending on each person.

From here, I write my opinion as Japanese native speaker. This is totally not linguistic explanation and My major is not linguistics.

Ending sentence with adverb without と or に is unusual (off course there are the sentences end with adverb without と(に). but most of them are I think in narration of TV or literature). So without と or に and end sentence with adverb, It makes me feel like the sentence end suddenly. Adding と in end of this sentence tells readers that this sentence omitted kind of the phrase like “やってきた”. When I recognize this sentence include omitted phrase, I don’t feel this sentence end suddenly and it feel more natural because Japanese often omit phrase.

Sorry my poor English.

My Japanese dictionary says that はるばる is an adverb and sometimes is used with と. So you are right — the existence of と doesn’t change the grammatical role of はるばる. In fact, はるばる and はるばると are pretty much the same in terms of both usage and meaning.

As background, sometimes adverbs in Japanese are followed by と or に.  (See https://kotobank.jp/word/副詞-123983; the 語形的分類 section in ニッポニカ explains this.)

For these words, adverb+と(に) is the wrong separation. “adverb+と” is one adverb. Whether you add と or not depends on each person.

The following is just my opinion as a native speaker of Japanese. This is totally not a linguistic explanation and my major is not linguistics.

Ending sentences with an adverb without と or に seems unusual (though of course there are exceptions, but I think most of them are in the narration of TV shows or literature).

So without a と or に following the adverb at the end of the sentence, it makes me feel like the sentence has ended suddenly. Adding と at the end of the sentence tells readers that there is something omitted at the end of the sentence, for example “やってきた”.

The と helps me recognize that this sentence includes an omitted phrase at the end, and as a result I don’t feel that it ends suddenly, making it feel more natural (especially in a language like Japanese where omitting things is common).