@benthegirl I agree that, after seeing the feedback, the first and third project look to be the most worthwhile. Plums are very tempting given how nice the fruit can be, but it sounds like there are a lot of challenges even when growing the native plums. If I had more land to experiment with, I would really like to try planting many goose plum seedlings to identify a highly disease resistant and pest resistant plum ( Seeds – OIKOS Tree Crops) to then cross with freestone plums that have some disease resistance to get an east coast friendly backyard plum. That being said, I probably will just focus on cornelian cherry and goumi.
For cornelian cherry, there have been breeding efforts in Ukraine and Poland. There is pretty good reference material on the characteristics of some of the varieties. A lot of information is outlined in the cornelian cherry topics: Sweetest Cornus Mas Cornelian Cherry? - General Fruit Growing - Growing Fruit and Cornelian Cherry - Yea or Nay? - General Fruit Growing - Growing Fruit. I think that cornelian cherry is generally hard to graft but there is reference material on the best ways to do it.
For goumi, I have not found a lot of breeding documentation and there is a more limited number of varieties available How many Goumi varieties are there and which is the sweetest? - General Fruit Growing - Growing Fruit.
I found a couple discussions on grafting goumi to autumn olive and there have been both successes and failures reported How many Goumi varieties are there and which is the sweetest? - General Fruit Growing - Growing Fruit. If you are interested in goumi anyway you could probably get a couple goumi plants and then try to graft cuttings of those plants to your autumn olives as the original goumi plants get larger (basically growing your own scions).
Seedlings, in general, have potential to find a variety that does well in your conditions so that is one cool thing about trying to grow out seedlings, though they likely will have more feral type traits because some of the traits of the named varieties are likely to be lost.