As someone whose first born was a stillborn (by late stage termination, at 29 weeks, so the cause of death was not unknown, the condition however was a tragic surprise very late in the game), I'm going to say the anguish will never diminish. My daughter is nearing 3, and my second son is 7 months. I miss my first child every single day. That doesn't mean my other children aren't being raised without 'full focus'.
I don't think my daughter knows her brother's name yet, but his hospital hat is on the wall with the other two kid's, along with their going home outfit- a gift my first son was supposed to wear but was much too small to be put into when we had photos taken. We have a birthday party for him each year, so eventually the living kids will remember that too. (My daughter still remembers his party this year, but just that we decorated cookies, she remembers the sugar, not the reason). We also attend a family event put on by a community non-profit where they do a butterfly release to celebrate the 'angels' in families around the community. I WANT my living kids to know about their brother. My daughter's first name and middle name squished together spell my first son's name in it, and my daughter and younger son's name do too (though that wasn't intentional). We honor him in many small ways. His name is also on my "mother's necklace" and he gets a stocking at Christmas (the gifts in it go to the children's hospital. We give him presents appropriate to the age he should be, and then donate them to a kid that age.)
He is not forgotten just because he never took a breath. He was real, and I held him after delivering him, stroked his head and sang him songs, counted his fingers and toes, and held onto his hand. His labor was the hardest of the three, as was his pregnancy. His nursery was in my house for more than a year before I finally opened the door and took it down. He was very very real.
So I don't expect Bella to feel second to Asa, but I honestly honor the fact that they want Asa to be remembered. The naming theme makes sense to me, even if I find the family insufferable. Bella isn't their first child, she's the second- so a B name makes sense. My daughter isn't number 2 in importance (she isn't 1 either, she and my living son are firmly tied), but she is number 2 in birth order. It's the same with Bella.