How difficult is it to return to active service after transitioning into the Reserves/Guard?

Here is the issue your husband can face. If he picks up SSG in the Reserves and then wants to go back to Active, he will have to face a Grade Determination to see if he could go back to Active as a Staff Sergeant. More often than not, folks don't get approved to go back to AD as a SSG. It can happen...just very rare.

I never went Active Duty but I did get mobilized for title 32 and Title 10 and then go on a title 10 tour (tour of duty/Mobcop). I went active duty for 4 years that way.

I have done it but I was an officer when I transitioned back. There are a few things you have to factor in. First, does the Army need Soldiers in that MOS? If not, then they don't. If they do then he has a chance. He will have to go to a rank assessment board. He is not guaranteed to keep his rank. If he were to go back into the Regular Army he may still get separated. For NCOs, you will have to go to a centralized board. If you are found "not fully qualified" for two years in a row the Army will separate the Soldier. That is for E6 and above. I would be concerned if he was promoted far behind his peers. That is the audience that the Army wants to separate if he gets in.