very busy - the fact that dd is moving her arms functionally and can get them to her mouth is a great sign at 8mos, definitely not a sign of cp.
(that isn't to say that she doesn't have any issues or whatever - too early to tell, but her arm movement sounds great).
For example, dd2's arms were locked into 'boxer' pose tucked under chin, thumbs in palm, from spasticity very early. We had terrible trouble 'unbending' her enough to get her dressed. If we laid her down, she would later 'bang out' to crucifix position, and we couldn't move her arms from there. She was unable to get her hands to her mouth at all, or grip anything unless we praised her fingers open (nigh on impossible) and wedged things into her palm. These days? She's 9yo and can use a knife and fork. 
It's so hard when they are tiny, but being able to get her hands to her mouth is a really good thing. Babies need to put stuff to their mouths to learn about the world, as well as part of normal oromotor development (leading to developments in feeding/ speech etc). Kids with cp who can't do this for themselves (and it's v common) have to have a lot of help - parents holding things to their mouth in an attempt to encourage mouthing, chewing, sucking etc. so this is not something you need to worry about.
When you say there is something unusual about her arm movements, what do you mean? A good physio will pick up even subtle stuff with movement.
At 8mos, dd2 had only been smiling for 6 weeks.
she was completely a newborn. She's doing v well now though, so for her it really was just a matter of time. It's very hard. When she was still in scbu, I used to say to the mum of a dd born on the same day, that if we could just have a crystal ball and see them on their fifth birthday, that I would be okay, because I would 'know' where they were going... I have to say that on her fifth birthday, I still had lots of unanswered questions, so the cp game really isn't a short term scenario, you just get better at dealing with it. Now I want to know if she's going to be able to drive, cope at university, find an employer who sees past the cp, find someone who loves her, have children.
You might not get a definitive dx for some years, but even if you do, there are no guarantees on what it means. Dd2 was dx spastic quad, and we were told she would be unlikely to walk or talk. I couldn't feed her, she screamed all the time she was awake, and I essentially spent the first two years (before she was even dx) teetering on the edge between coping and sobbing. Presentation can also change over time. Dd2's dx was changed at 4yo, we requested another MRI as her tone was completely different to original dx.
That said, at 9, she walks, talks, runs, sings (badly), has always gone to ms school, goes to ballet, swimming and brownies, is learning to ski, wants to be in the Paralympics, and is currently writing a book about having cp. (she had taught herself to read before she could talk... But obviously we didn't know that!)
Right now, none of your questions are going to have quick answers. You need to be kind to yourself, and buckle in for the long haul. So it's about getting coping strategies in place for you. (Ultimately I ended up in counselling when dd2 was 6 or 7) making sure you have somewhere to offload, and a dp or friend or nursery or childminder that can take dd for an hour or two here and there so that you can try and relax. Get out of the house, if you can't bear a regular mums and babies group because the differences are too stark, find an sn group. From the sounds of it thou, you should be okay at a regular group, and you may find it interesting. I used to go to both. The more I was out of the house, the better for my sanity. It was hard to fit it in between appointments, but I knew it was the only way to stay sane.
the first two years are a bit grim. Did they prescribe you anti-ds?
If you are curious about reflux, why not pop to the gp and discuss a trial of infant gaviscon?