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Your plants should be much bigger by now. I would dig them up and plant them in a container with potting soil and grow them inside in a sunny location.

Before you plant them outside in the spring, add some compost to the ground. It's possible the soil is very clayey and preventing the roots from growing.

https://www.gardeningknowhow.com/ornamental/flowers/foxglove/foxglove-flowers.htm

Foxgloves are biennial, so the first year you only get foliage, the second year they bloom. When they do bloom, if you want them to reseed for the following year, stop deadheading the plants in late summer so they can go to seed, or at least leave a few plants to go to seed.

https://www.gardeningknowhow.com/ornamental/flowers/foxglove/foxglove-flowers.htm

Nope! A biennial is a biennial, unfortunately. You can plant seeds, though. This article will help:

https://www.gardeningknowhow.com/ornamental/flowers/foxglove/foxglove-seed-harvesting.htm

As long as you keep it out of direct sun, and as long as the humidity remains on the higher end. Really, it is easier to care for them indoors, but outdoor growth is possible with the right preperation.

This article will help you to care for the plant:

https://www.gardeningknowhow.com/houseplants/streptocarpus-plants/care-for-streptocarpus.htm

Unfortunately, this is the issue that comes up with overseeding. In most cases, they will struggle after trying to pull them up and transplant them. This isn't to say that you can't try and save them, but this will likely be the case most of the time.

Composting them is not a bad idea. This is common. You can continue to do so.

Here are some articles that will help:

https://www.gardeningknowhow.com/ornamental/flowers/foxglove/foxglove-flowers.htm

https://www.gardeningknowhow.com/ornamental/flowers/delphinium/delphinium-seed-planting.htm

The best time to plant new seedlings is in the spring when there's no longer a chance of frost. If you can anticipate that time, you may want to start your seeds indoors in late winter so that your seedlings will be ready to put into pots in spring.

If you want to direct sow, the seed packet will tell you when to plant according to your region.

Here's an article with details about how to start your seeds:
https://www.gardeningknowhow.com/garden-how-to/propagation/seeds/when-to-start-seeds.htm