You married to him so you know his personality better than anyone else here. Would he be the kind of guy who would cheat? If the answer is no, there is no need to be obsessed about it. If the answer is yes I don't think you would marry to him in the first place. I am not sure about you. I can tell someone who could possibly cheat or not within 3 months of dating. Men cheat for different reasons. Some are born to be cheaters. Some want out because they are not happy with their marriage. The later apparently doesn't apply to you.
Normally a man would not take the risk to date coworkers unless there is 95+% match. He knows her before you and he didn't pick her. Would a man flirt a work? Absolutely.
They work together for so many years so they are friends. She may have some crush on your hu*****and especially back in the days when he was single. Now he is married to you she would keep her crush at the flirt level. You can not control that so just let it go. Your hu*****and told her that you were beside her when she called because he was afraid that she might flirt with him. And your hu*****and wanted to make sure she behaves when you are around.
He is not avoiding the problems. He just didn't think there is a problem.
There would only be a problem if your hu*****and lied about her looks. One of the days you would meet her at your hu*****and's company Christmas party or something. If she is in great shape and beautiful that might be a different story. But I highly doubt that.