scale_ratio is the final height in pixels of the emoji because 72x72 pixels is too big, You'd have to scale it but you can set 9 so Minecraft will handle it automatically.
y_position is the y position in pixels, 8 should be good if you keep emoji 72x72 and scale_ratio to 9. If you have different size you'll have to tweak the y_position based on your tests.
scale_ratio must ALWAYS be higher than y_position.
I cannot fix that issue, it's a Minecraft limitation.
In-game view
Using the /e useful suggestions
Write /e in chat, press SPACE and press TAB to start autocompleting.
Using the tab autocomplete in chat
Write : in chat and press TAB to start autocompleting.
Download examples:
Unexpected error with integration github-files: Integration is not installed on this space
Create big preview on hover
You have to create a new font image for the big emoji (you can use the same .png image) and set scale_ratio to a big value, usually 64 is good.
Edit your small emoji by adding the chathover property with the custom override_text.
The override_text is used to show the big emoji in the hover description text.
Note: the \n part is important in order to match the big emoji image height, otherwise the tooltip will be too short.
Adding a title
You can add a title, just write your text and add a \n character to start a new line.