Is it permissible to feed the poor instead of making up 45 days of Ramadan fasting due to pregnancy and severe bleeding, as ruled by Ibn Abbas (may Allah be pleased with them both)?

What the Sheikh told you about Ibn Abbas is correctly attributed to him, but it goes against the view of the majority and against what we issue fatwas on. What is binding upon you is to make up those days only. You are obligated to make them up whenever you are able to do so. You are excused as long as you are unable to make them up. So, when you are able to do so and have regained your health, then make up what is binding upon you to make up.