Should be fine.

聽說即使有Reentry permit 超過一年回來也會有 沒收綠卡的危險,

-Air accident sometimes occurrs, but it does not prevent us from our travel by air, right?.

而且三年內他們每年都回去住4-5個月.他們很擔心.

我想讓他們不超過12個月回來一次,住1-2個月,回去後,在reentry 過期前回來. 能保證不沒收綠卡嗎?

-Not need. The reentry permit functions for the GC holder to stay outside US for less than two years.

我父親有心髒病, 可否以看病為由,說明不得不在國內看病?

-Yes. It is a good reason if asked.

因為我們州必須滿 2 年才能申請medicaid.
非常謝謝!!