Are multiple separate trips of less than six months outside the U.S. deemed abandonment of permanent residency?

The purpose of the absence is critical. Immigration has been understanding when a student attend school and in the past, I would say do not worry. Now I say still get a reentry permit.

Certainly, it is the safest to do a re-entry permit if attending school or working overseas to make sure you are not determined to have abandoned status. Until you can return to do that make sure to file taxes on your worldwide income, maintain a residence and other ties here and do not stay away for more than 180 days. It is a factual determination as to intent to abandon so no clear rules giving the CBP Officers way to much discretion.

Based on your narration, you are fine. However, considering you still have two more years of study to go, you should seriously consider obtaining a reentry permit.

CBP can do all sorts of things, but if you otherwise have established that the U.S. is your permanent home (and USCIS lists several factors), you may be able to get through secondary. I believe you will be subject to secondary with this proposed travel plan. A reentry permit may still be a good idea.

For good measure, apply for a re-entry permit. Short trips generally should not amount to abandonment, but the frequency can also be a factor.

Without a re-entry permit, you are taking a risk, but likely, you will not be deemed to have abandoned your permanent residency if you can show ties to the United States and convince the CBP officer that your schooling abroad is temporary.

Generally not. But you should apply for a re-entry permit. It would allow you to stay outside the U.S. for up to two years at a time.

It is possible that the pattern of travel can show an abandonment, but there is only a presumption of abandonment for trips over six months.