For the Q3 : you can adapt the size of each emoji using this CSS
img.emoji[title=":haha:"] {
width: 20px;
height: 20px;
}
They’ll always appear weird on the selector but in the posts, it’ll be at your taste.
Also, here’s my emoji plugins, you can use it as an example : you can sort them as you like without any alphabetical order, they’ll be in the custom section on the selector, after the uploaded emojis.
The default plugin was made by @zogstrip, you can find it here : https://github.com/discourse/discourse-emoji/