Should be fine.

听说即使有Reentry permit 超过一年回来也会有 没收绿卡的危险,

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

而且三年内他们每年都回去住4-5个月.他们很担心.

我想让他们不超过12个月回来一次,住1-2个月,回去后,在reentry 过期前回来. 能保证不没收绿卡吗?

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

我父亲有心脏病, 可否以看病为由,说明不得不在国内看病?

-Yes. It is a good reason if asked.

因为我们州必须满 2 年才能申请medicaid.
非常谢谢!!