Yes, you can return to the US at any point with valid GC and reentry permit.
When it comes to naturalization in the US, if your husband spends 6 months or more outside of the US, then he'll have to wait full 3 or 5 years again to naturalize.
I'd monitor closely the changes in German laws. If it's not urgent and you can wait for 2-3 years, maybe he should naturalize first once German laws get passed. The kid will become the citizen by operation of the law. Then you're all free to go to Germany and live there however much you want. And won't have to deal with USCIS ever again.