Those are Damsons, or possibly a wild plum/Bullace. Certainly not a sloe. Usable in alcohol either way.
To keep the envelope and contents flat so that it does not exceed the thickness limit by bunching together, I use a tiny piece of double sided tape to hold the seed envelopes in place inside the posting envelope.
Important thing to do though is to remove all the plums, when ripe and burn or bin the infected ones. That reduces the population a little.
Transplant when young, they hate root disturbance and more often than not die if moved when bigger plants.
I live on frozen apple purée and frozen baked apples in the winter. Well at least for desserts anyway.
No, as you can see it works for me.
Picked ours last week, ripe too. As suggested, purée and freeze for winter use or if unbruised by the fall, core, fill with dried fruit and syrup and bake in the oven. Then freeze for winter use.
They needs food. They needs sun. They needs planting with the top of the 'bulb' no more than an inch below the soil level. So, yes move them to a better position.
Now called Ageratina altissima 'Chocolate':
Probably not getting enough food to flower.
The leaves on P. obovata are more rounded than the ones on this to my mind.