I am a first timer too and have really suffered anxiety terribly since having my DD. Part of the reason I was in such a state about returning to work was precisely because I had hardy ever left DD at all with anyone for the whole year, all because of my anxiety!
I sympathise massively with how you are feeling and would find your situation regarding the grandson and what had happened on your 'surprise' early arrival hard too.
Not to want to 'take you MILs side' but, a few things....
If someone snuck in on me and DD at the wrong moment (like for example, if I have to put her in her playpen when she doesn't fancy it because I have to go to the loo!), they could find her crying her eyes out with me out of the room and I absolutely hate letting her cry and only have her to look after.
Also, when my MIL had our DD all day for the first time, she followed her schedule (that I had written out in excruciating detail ) to the letter. I know this for a fact. DD who had been finally, sleeping through really well, did not sleep AT ALL that night, to my horror and her sleep pattern went totally mental for a while after that.
When my DM had her she went rather 'off road' shall we say, regarding the schedule (she is much less organised than MIL but every bit as loving and loads of fun ). This I also know for a fact. DD went straight to sleep and stayed that way till morning!
Loads of stuff will disrupt your LOs sleep pattern over the next few months and you can put some of it down to your MIL but your DDs schedule will be evolving anyway. DDs sleeping is back on track btw, thank God after just one desperate early morning episode of CC which nearly ripped my heart out but was totally worth it.
The last thing I wanted to say, which is going to sound awful but you will see what I mean in 2 or 3 months.....is that a 2 yo child is so much more engaging than a 9mo. He is chattering away (whether your MIL likes it or not!), running about and generally guzumping your DD in the attention demanding stakes! He has had over a years head start to get practice afterall!
But, your DDs character will blossom so much over the next couple of months and this will help your MIL forge a stronger and frankly more interesting and pleasurable relationship with her. Soon she will be calling your MILs name, toddling around and beaming her even bigger toothy grins than she probably does now. I know that my DDad for example, is relating to and showing tons more affection, interaction etc with our DD now that at nearly 12m, she is so full of personality and mischief character.
I completely understand you wanting her to be treated like a princess, believe me I do but I am just thinking of the things that I tell myself when I have felt a bit wobbly about childcare issues and I always end up reminding myself how lucky we are to have people who actually LOVE our DD looking after her. Don't forget, your MIL probably doesn't 'see' YOUR daughter as much as she sees her SONS daughter, her own blood.
I would say try really hard to take your first day first and then go with 'baby-steps- from there.
Keep communication with your MIL good as possible, tell her how awful you are feeling about returning to work (not due to her childcare!) and how glad you are that DD is with her Grans. Some positive comments might go a long way. Tell her of problems you encounter sometimes and seek her advice, she may feel more inclined to open up to you rather than hide things then if she knows that you find it difficult sometimes too. She might already have a feeling that she's not 'up to your standards' which is not a positive foot to start off on. Sounds to me like she's really got her hands full, but I know how hard it is for you envisaging all kinds of awful scenarios. I've done it myself, imaging our DD putting all manner of dangerous items that are dotted around my parents house, in her mouth (little piles of safety pins, tweezers, loose change, pens, screws......'a place for everything and everything in it's place' means nothing to my parents, God love them ).
Even as worked up as you are feeling, you have said that 'you know she loves her to bits' and that counts for a hell of a lot in my book. Keep talking to her, keep a close but balanced view on the well-being of your DD, take it one shift at a time and give it a chance.
p.s. Mobile might be the way for you to go in the future but you will lose the camaraderie of your colleagues. See how everything works out before you make any rash decisions.