Thank you for your indepth review and im glad you enjoyed it.
Most of the enemies that ignore the lanes are intended. Only the groups of 4 fairies that do actually arent intended. But i do think that one is a positive accident.
I will try to add some indicator in the post jam version for when new lanes get added. There wasn't enough time to do something like that but there is a tip that tells you to always look around for new lanes that may pop up.
The balancing of the plants and enemies does need a lot of balancing but there wasnt much time to playtest to balance how much everything should cost compared to how useful it is. There will be more maps in the future that use all directions for enemies so plants like the daisy and cosmo will be more helpful in those.
The plants are indeed planted on a grid. It's so you can't place multiple plants in the same location (which would look even more awkward). And it also just makes things simpler to manage.