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They produce a lot of bulblets so you may not need to split them, but if you do, I suspect they will grow just fine. They are very vigorous growers and most people want to know how to get rid of them. They also reproduce by seed.

I've come to the conclusion that this a species of aphids! They aren't pale or brown/orange like the average aphid, but are black and known to like onions. So, I'm giving you a link that explains how to eliminate them naturally, and hope this helps: https://www.gardeningknowhow.com/plant-problems/pests/insects/homemade-aphid-control.htm

It means you shouldn't plant seeds in the ground or your seedlings outside till all danger of frost has passed. You already have a jump on the season by starting your indoors.

I think you tried the wrong type of onions. Walla Walla don't tolerate below -10 degrees F. The article on winter onions below says to use bulbs and gives some popular varieties. They also must have full sun. If you pull them indoors, they probably need supplemental light. Here is more:

https://www.gardeningknowhow.com/edible/vegetables/onion/growing-onions-in-container-gardens.htm
https://www.gardeningknowhow.com/edible/vegetables/onion/growing-winter-onions.htm

Full sun. As much as you can give them, but at least 8 hours of full sun is required.

https://www.gardeningknowhow.com/edible/vegetables/onion/find-how-to-grow-onions-in-your-garden.htm

To make things easy, you could purchase "raised bed soil", which would suit your needs.Otherwise, potting soil amended with extra compost would be good, as well. There are other ways to go about this as well. After you get the basic blend down, you will need to amend the pH to the specific plants that you will be planting.

Here is an article that will go into this, further:

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/best-soil-raised-garden-beds.htm