This is actually somewhat similar to other threads, and it seems that it's pretty evenly split as to whether it's okay or not.
On one hand, there are those who think you should offer food/drink if you are going to indulge, even if you know that the other person will decline. For instance, even if you are fairly sure that Mary isn't a fan of chocolate, you would ask her if she wanted one of the truffles you were eating as a way of acknowledging that you wanted to include her and offer your hospitality.
On the other hand, there are those who think it is rude to offer when you know that they will say no. (The reasoning is that you are perhaps demonstrating that you are not paying attention to their dietary restrictions and preferences.) If Mary is a vegan, consistently offering her butter for her potatoes could very well send the message that you weren't listening when she told you that she didn't eat animal products. It's one thing if you forget once or twice, but if you eat with Mary several times a year and she has told you that she's a vegan five times already, continuing to ask isn't necessary and could grate.
I would say that it is probably best to not have dessert if you don't have something for everyone, even if it is just fresh fruit or mints for the non-pie-eaters. I also think your mom should have offered coffee and tea automatically. However, if she was fully aware that everyone would decline, I don't think it's necessary to go through the formality of offering it. As others have said, though, she made other progress with the chicken, so take it as a baby step forward.