Any patch of dirt with no opaque block above it (so only air, leaves etc) will grow grass after a while. If any block is covering it, blocking out the sun, then the block will remain dirt.
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You cannot get grass seeds in Minecraft. However if you want to plant some along with some flowers you can use bonemeal on some dirt/grass and it will grow.
No, but make sure the tree has enough light as well as space to grow. Also saplings can only be planted on dirt or grass blocks.
Grass, Probably.
Kittens don't grow in minecraft
It doesn't 'spread' to anything, but will grow naturally on dirt which is placed in a Large Mushroom biome, like grass grows on it in other biomes.