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.