I don't see that you have any obligation to go the funeral. It doesn't sound like you're close to the kind of people who would try to bully you into going (gasping in horror "But she's your moo!") and who would talk shit about you if you don't go.
Frankly, they don't sound like they're taking your feelings into consideration or being supportive to
you. So, when deciding whether you want to attend the funeral or not, I would not consider the feelings of the kind of people who would talk shit about you if you didn't attend. Decide instead based on what would be best for
you. I say that not really knowing much of your situation or these people, only what you've said here, so it's just my opinion, and I hope this isn't offensive.
They shouldn't be trying to make you feel badly about not going, and you don't owe them an explanation either way.
Funerals are for the living, in my opinion. People may think it's disrespectful to your moo if you don't go, but she'll never know the difference. People may says it's disrespectful to the grandmoo if you don't attend, but her feelings aren't more important than yours. What I'm hearing from you is that none of these people have considered your feelings as much as they have their own, and your feelings count, too. Consider going if you feel it would help
you in some way.
There must be a significant amount of pain inflicted on you over the years if you're considering not going. I can totally understand not wanting to go to a funeral where she will be honoured and mourned as a wonderful moo and wonderful person if you're feeling a completely different type of grief, the grief over not having the kind of mother you needed. (Pardon me if that's completely not the case; it's just the way I read your situation). I can also totally understand not going if you've just had all the grief you can take and the funeral is just too much emotional overload and famblee stress.
I would say remember that your feelings count too, not just grandmoo or anyone else. Maybe consider if there was someone else going who you do want to support, maybe your duh or someone else, and who would be equally supportive to you. But if you don't want to go, or if it's just too much for you, and you're feeling pressured to go just for appearances sake ("But she was your mooo! What will people think?") or being pressured to support someone who wants to make it all about themselves (grandmoo?) then I would step back and take a breath and consider your own best interests before committing to going.
Anyway, apologies in advance if this comes off as offensive or if I've read your situation completely wrong. I'm just trying to say that your feelings count as much as anyone else's, and that no one has the right to use guilt to pressure you into doing something that isn't right for you. Only you can decide that. And you won't be "wrong" either way. It's a tough thing you're going through.