As promised, I've returned to play the game. Unfortunately, there are quite a few issues.
I frequently encounter the bug you pointed out about how the characters can't navigate around each other. This unfortunately stopped me progressing past level 4 at most.
If you purchase the second or third room first, any room previous will never be entered by the characters in any following level.
The third 'Buy' button's click detection isn't the size of the button itself. You can see this simply by hovering over the top half of the button.
The play button is really stretched and doesn't look very appealing, same with the health bar.
Not being able to defeat the character on level one is rather unattractive, it means you are always going to lose health no matter what.
I can't really comment on anything else as I frequently could not get past level 1 due to the characters getting stuck on each other.
I do like the art style and the idea of the game. The mechanic of the game reminds me heavily of a card game I recommend you look at if you are interested in taking this project further. The game is called Boss Monster.
You have a really promising game, but unfortunately it is just unplayable in it's current state. If you were to fix what I have suggested, I would love to replay and give you further feedback.