The greengage plum produces firm fruit with flesh and skin which remains green even when ripe. If you want the tree to produce a crop of fruit for you, maybe you should research any pruning techniques which need to be used to stablize your prune tree and to help it produce its best yield of fruit each year. Also, prune the trees near it in a manner which will provide your prune tree with the right amounts of light that it needs to successfully produce that good crop of fruit. It certainly is good that pests do not bother the tree and fruit.

I have Eureka, Ponderosa, Meyer and three seed grown lisbons. If you want the tangier more acid flavor then by all means go with Eureka or Lisbon. Eureka does better in cooler climates and Lisbon in warmer climates. Ponderosa as mentioned is a hybrid but still has very good lemon flavor but like it parent the citron is more pulp and peel than juice. Meyer is great for lemonade and lemon pie but don't be fulled by it's higher sugar content. You will still have to add sugar when cooking with this variety. Keep in mind that Meyer usually doesn't store as long as true lemons and it has been my experience that the flavor is better when used fresh. The quality of fruit may be impacted by the rootstock it is grown on. Sour orange and trifoliate orange for colder climates are usually the industry standard and have varying degrees of disease tolerance. Enjoy your lemons!!!

"Sorbus alnifolia is one that is supposedly borer/disease resistant," I just saw one at the National Arboretum that looked disease free, however I had to remove one in my garden that I got from Arborvillage just before they closed because one year, after going for several years w/o it, it got a huge case of fireblight. Almost all of the top turned orange. It managed to regrow later in the summer and I doubt it would ever have died of it, but I didn't want a plant in the garden where that could happen. Imagine in 20 years when it was full sized - how hideous that would have been to deal with. When I went on a googling binge of Sorbus years ago, there seemed to be much more hybridizing and selecting going on in Europe. There was some promising Asian X Asian or Asian X European hybrid, fairly easy to buy in the UK, that looked very promising. Advertised as completely resistant to everything they encounter there (which may not matter here, granted) BUT it wasn't listed in NALT, wasn't in the UMN plantfinder, didn't have any google links in the US so I assume it was never imported. Looked like a conventional mountain ash, too. Longwood Gardens has a selection of one of the Asian species but it's really hard to find. IIRC Forestfarm was sold out of it a few years ago, I should look again I suppose.

Regarding the acorns the caps need to be included to get the full picture. But I also thought the foliage looked like that of black oak so that is two votes.