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It is very hard to say, since different nurseries may or may not use different root stocks. It would be another type of Ash, though.

Still, that growth from the rootstock is heavily infected, as well. Those trees will not, likely, survive.

I notice that the soil is pretty much pure clay. This will be the reason behind the planting failure. The soil needs to be amended with PLENTY of organic material. Heavy clay soils can be bone dry on top, but saturated just an inch below the ground level. This means that whatever is planted in it (unless it is designed to handle clay) will suffer from suffocation and root rot.

Once you amend your soil, and kill off the infection, you can start with fresh trees and proper care.

Here are some articles that will help:

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-soil.htm

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/soil-ph-plants.htm

https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm

https://www.gardeningknowhow.com/special/organic/biofungicide-information.htm

https://www.gardeningknowhow.com/garden-how-to/info/homemade-fungicide.htm

https://www.gardeningknowhow.com/garden-how-to/info/applying-bactericide-to-plants.htm

https://www.gardeningknowhow.com/special/organic/the-importance-of-soil-for-an-organic-garden.htm

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/improving-garden-soil.htm

https://www.gardeningknowhow.com/garden-how-to/projects/how-to-solarize-garden-beds-to-eliminate-garden-pests-in-the-soil.htm

https://www.gardeningknowhow.com/ornamental/trees/ash

I wouldn't recommend sterilizing a tree, yourself. This will require a professional that is knowledgeable on the safe use of the chemicals required to sterilize a tree. They pose a very significant risk to humans, and are not usually worth the damage that they can cause.

Removing one of the trees will not solve the issue, either. The large female can receive pollen from trees nearby, assuming that there are others within pollination range outside of your yard.

Should you still want the tree sterilized, you can contact your local extension service for help with this. Here is a link to help you find the closest to you:

https://www.gardeningknowhow.com/extension-search

You can. This article will help: https://www.gardeningknowhow.com/ornamental/trees/tgen/grafting-trees-what-is-tree-grafting.htm

Yes, this would be ideal in this situation. Once treated, the new tree should be just fine. Just make sure that you take preventative measures against borers in the future.

Here is an article that will help you to keep them away:

https://www.gardeningknowhow.com/ornamental/trees/tgen/tree-borer-insects.htm

It will have the same care as the original. Suckers are a little more hardy than a sapling, or seedling would be. They are attached to a very large root system for their size.

If it requires staking, you may need to look into soil fertility. Lack of silica will make a plant or tree flimsy. These trees should not need to be staked.

This can be corrected with potassium silicate, should a soil test show the need. Just make sure to start slow. Start with 1/4 strength and work up to half strength. Too much and you risk locking out potassium to your tree.

Here are some articles that will prove useful:

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-soil.htm

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/soil-ph-plants.htm

https://www.gardeningknowhow.com/ornamental/trees/ash

It could be a European ash.

https://selectree.calpoly.edu/tree-detail/fraxinus-excelsior