Your drawings are peaceful and adorable. 
It’s great that you’ve considered the ‘red flags’ and know that this is a good project for you. It will be a lovely book for your friend’s family to enjoy.
I agree that it’s important to decide who will handle the print order, then choose a specific print service & evaluate the options from that vendor to finalize the size requirements, so you don’t have to worry about future adjustments.
It’s great that you are seeing the issues now in the thumbnail stage. That’s what the thumbnail stage is for!
I like your character design in the 3rd image (with just the mouse and no background). I agree with the other comment that you can see the cute little face better & the proportions are better than in the full image, but I also love the background with the dandelions.
I don’t think you need to worry about “merging the style of [your] characters with the environments”. I think your images look good. There are plenty of examples of work where the background is more painterly while the characters aren’t. So if you like that style, go with it.



