L.R.
This would not really be such a big deal with you if your child responded to and listened to you, would it? It's about power: Your mom has had the authority over your child and now you want it back. But you cannot just seize it after being away for what amounts to all the time your child can remember.
Please give your child time. She is VERY young still -- you do realize that, don't you? And you do realize that kids go through many stages of calling people and things many different names, and those stages change?
You also hae been away for eight months. To a child of less than two years old, eight months feels like her entire lifetime. She very likely cannot even remember anything that happened eight months ago -- so frankly, to her, you are a nice and very loving person who is in her life but not the person who has fed her, put her to bed, disciplined her, done fun things with her, made her do stuff she didn't really want to do -- in other words, parented her -- for all the time she can remember. Yes, you are her parent; please don't get wound up that I'm saying you're not. But young kids only know what they experience. Telling her a thousand times "I'm mommy" is not going to make you mommy to her. That takes time and your constant presence. You're back now and that's great, but if you focus on names and on your own hurt feelings -- you are going to blow this by making your mom angry and confusing your child.
Of course this hurts you and makes you sad, but you need to step back from your emotions here and put her first. I'm going to say that again: You need to put your own feelings on hold here and think like your young child is thinking -- you cannot shame her or blame her or scold her or she is going to run for comfort to....your mother.
Your child is NOT being intentionally disrespectful but is only acting on what her very young mind understands: "mommy" is the one who has been there daily and "boo boo" is this sweet lady who gives her a lot of attention just now. And please think about this: She hears YOU refer to your mother as "mom" so she is following suit since to her that is your mom's name.
Get your mom on board first and foremost. Talk to her with your child nowhere nearby and explain gently -- not defensively or angrily -- that you're hurt; and that you also realize that this is no one's fault, just a fact that comes about because you had to be away. Ask her for her help: "Mom, it is important to me to have Kid call me mom or something close to it. When she calls me 'boo boo' I'm not crazy about it but I do get that it's hard for her. I'm just asking you here not to encourage 'boo boo' but when she says it, to tell her, that's your mommy. If she calls you mommy too for a while that's OK but for now, please help me out by downplaying 'boo boo' and not raising it in front of family or encouraging it with Kid."
Then you DO have to be OK for a while with hearing your child call her mommy. If you hate being Boo Boo, then tell your daughter to call you Mama and leave mommy for her to call grandma - your child will get the difference. When she says Boo Boo, smile (don't scold) and gently say every single time, "Who's this Boo Boo? Only Mama here," and so on.
Again, please realize that this is not actually about the names; it is about both you and your child readjusting to each other after so long apart. Even if you visited her a lot while a school -- that's not the same to her as living in the household. You and she both have to adjust. If you can move out with her and not live with your mom eventually that will help her to adjust to you as mom, but also do not jerk her suddenly out of the home where she has done nearly half her life's growing. I'd read up on child development for her age and ensure that you do not ever let emotions or conflicts with your mom get in the way of your kid's needs. This is not, repeat not, an issue for discipline, and it is not an issue of respect for you from your child - though it will be an issue of your mom's respect for you if your mom cannot stop making it a family joke.