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Full sunlight indoors is likely not enough sun for your plants.
These plants need 6 plus hours of direct full sunshine.

If you can't move them outdoors you could add a grow light to add additional grow time.

https://www.gardeningknowhow.com/houseplants/hpgen/what-are-grow-lights.htm

Some hybrid flowers are bred for frilly, double or triple petals and lack the core when nectar, pollen and seed are produced. These flowers offer nothing for wildlife. When flowers hardly resemble the natural plant due to extensive breeding, bees may not recognize it as something that is safe and beneficial. Instinct tells them the "Teddy" sunflower is not for them.

The way I do it, is to cut down the blooms when the petals fall off, and hang them upside down until the seeds start to pop out. Then you can pull out the rest of the seeds. Here is an article for more information: https://www.gardeningknowhow.com/ornamental/flowers/sunflower/harvesting-sunflowers.htm

That depends on your area. You will want to plant these when the soil reaches at least 60 degrees. You will harvest when the flower petals fall off, and the head droops forward. Here is an article that will help: https://www.gardeningknowhow.com/ornamental/flowers/sunflower/harvesting-sunflowers.htm

Check with your local garden centers for decorative stakes or regular garden stakes.
Search on Pinterest for some interesting ideas!

The insect in your photo appears to be a 'leafhopper'. But I doubt that this is what is causing holes in the leaves and stems. Leafhoppers are sucking insects and not chewing insects. There may be grasshoppers or another insect pest causing the damage you describe.

The following articles may be of help to determine the actual pest causing damage and the appropriate control.

https://www.gardeningknowhow.com/ornamental/flowers/sunflower/sunflower-problems.htm

https://www.gardeningknowhow.com/plant-problems/pests/insects/leafhopper-control.htm