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Fertilizer should not be applied directly in the planting hole, because fertilizer can burn the tree's roots. Instead, you can add fertilizer the following spring:
https://www.colostate.edu/Dept/CoopExt/4dmg/Trees/command.htm

Compost can be mixed in with the soil around the tree, but also shouldn't be used unmixed in the planting hole. It is also fine to not add any compost or fertilizer.
http://www.clemson.edu/extension/hgic/plants/landscape/trees/hgic1001.html

You do have a few options here. You can attempt to graft the branch back on using this article as a guide: https://www.gardeningknowhow.com/garden-how-to/propagation/grafting/reattaching-broken-stems.htm

You can plant each individual cutting as a separate tree. This article will help you here: https://www.gardeningknowhow.com/ornamental/trees/tgen/planting-trees-from-twigs.htm

And if none of those methods work for you, the tree will sprout new branches on its own. It may take it a little longer but it will likely recover.

You can, usually, plant the tree just about any time of year, as long as the soil remains moist. I would make sure there isn't a freeze in your area at all this week. But other than that it should be ok. This article will give you more information: https://www.gardeningknowhow.com/ornamental/trees/dogwood/caring-dogwood-trees.htm

Black knot infection typically occurs on trees in the cherry/plum genus, Prunus. Black stem disease affects dogwoods. Ugh! Read up on black stem disease and see if the symptoms match your dogwood. In either case, treatment consists in removing infected branches, cutting a good 6-8 inches back from the knot. Clean you pruning tool between cuts. Normally this is done in the dormant season before leaf break. Dogwood pruning cuts are susceptible to several diseases. By the time black knot is visible, it has been growing for at least a year. That means there are spores nearby ready to cause new infections this year. Using a fungicide rated for black knot and for dogwood prevents these lingering spores from starting a new knot. https://www.extension.umn.edu/garden/yard-garden/trees-shrubs/black-knot/

After the bloom cycle would mean when the tree is done blooming.

Your tree seedling should continue growing after moving up to a slightly larger pot.

You can place the young seedlings into a bright but indirect sun location in the garden that is sheltered to harden them off.
You can gradually move the new seedlings to a sunny location.

Why is the tops of my dogwoods dieing?