Whoops! Yeah, the game does lack a bit of a tutorial so I don't blame you.
And also a bit of a mistake in my explanation - for every level, every enemy you place gets more score (first 1 point, second 2 points, third 3 points, etc). The amount of enemies is limited by the available tiles, so this means that there are more points to score in a level with 100 tiles than in a level with 2 tiles. The score per enemy throughout the game does stay the same.
As for the generator making a straight line - yes! that is possible. Although, to make it truly impossible, that would also mean that the player has to spawn on one end, and at least one button at the complete other end. if there's space outside the 'line' between the player and the farthest button, an enemy could be placed there, but that would also likely have to be a green enemy.
I'm sure there are some calculations that could be done to calculate the chances of that happening but in any case, it is possible!
Alright, ramble done, haha