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Yes, likely the fertilizer did give it what it needed. You can fertilize in the spring.

Rhubarb can be fertilized when the stems get to be about a foot tall and you can keep harvesting (though don't take all the leaves) until late June.

There are some varieties of green rhubarb. The stems don't turn red when ripe or anything, so if yours always stay green, you may have a green variety.

This article will help with growing rhubarb: https://www.gardeningknowhow.com/edible/vegetables/rhubarb/grow-rhubarb.htm

When the stalks start turning red or magenta in color, it means that the rhubarb is ripe and ready for harvest. In addition, their size should be around 10 inches or so. Here is an article that you may find helpful: https://www.gardeningknowhow.com/edible/vegetables/rhubarb/harvesting-rhubarb.htm

It can be harvested, but you may find the stems to be a little tough. There is no harm in trying though.

Yes. This article will help: https://www.gardeningknowhow.com/edible/vegetables/rhubarb/grow-rhubarb.htm

It is likely slugs. They come out at night and are rarely seen on the plant during the day. This article will help:
https://www.gardeningknowhow.com/plant-problems/pests/facts-about-slugs-and-how-to-kill-garden-slugs.htm