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They are designed to go for that long. Although it can go for that long, the fruit size, quality, and quantity will diminish greatly. I would recommend with starting new crops every season. This will keep fruit production strong.

You should definitely dress your plot each year with dolomitic lime, wettable sulfur, and plenty of organic matter such as compost, and worm castings. This will ensure that your soil remains completely fertile after each season.

I have always grown my eggplant plants in large containers with great success. I fill the soil in the pot to about 2-4" below the pot lip. It is also very important to use a well draining container. For more information on container grown eggplants, check out this article:

https://www.gardeningknowhow.com/edible/vegetables/eggplant/container-eggplant-plants.htm

The only problem come when they are directly touching roots in soil. Growing them near each other will not cause any effect at all. The incompatibility has all to do with the soil zone, and not proximity. You will be fine to plant anything near each other, as long as they are in separate containers.

If they are coming out close to the base, then yes you can. If they are higher up, then they will be fine left on.

The lower growth will sap nutrients from the rest of the plant, so removing them will help in the long run.

This article will give you more information on the care of these: https://www.gardeningknowhow.com/edible/vegetables/eggplant/growing-eggplant.htm

It could be a pollination issue. You can take a paint brush and gently move the pollen from one flower to another.

https://www.gardeningknowhow.com/edible/vegetables/eggplant/pollinating-eggplants-by-hand.htm

https://www.gardeningknowhow.com/edible/vegetables/eggplant/container-eggplant-plants.htm

This article will help you.

https://www.gardeningknowhow.com/edible/vegetables/eggplant/how-to-harvest-eggplants.htm