Unfortunately, I doubt many will see your post in this obscure photo sub-forum. Also, I am a little uncertain about some wording in your post, but I think I understand the general idea. Trees can often cover over their wounds if they are growing vigorously and are in good health. You mentioned that your tree has been "pruned to be very sparse". That sounds like you are making it much harder for the tree to successfully recover. Starving the tree of food by limiting it's foliage and forcing it to overcome pruning wounds would certainly present obstacles to the tree's recovery. It is unlikely that removing a section of trunk would be the way to go. A bridge graft might help, but is probably not worth the trouble. Again, what you need to do is to increase the tree's vigor, and the best way to do that might be to leave it alone for a while! And, yes, the plaster cast is a crazy (bad) idea.

Japanese maples grown in a container in your zone must be overwintered in a protected location. That means a garage, unheated basement, greenhouse, etc. Just wrapping the pot is insufficient protection - temperatures that drop below around 26F can fatally damage the root system. Otherwise, a lot of Japanese maples should do well in your zone in the ground - zone 5 is generally considered the lower cut-off for these trees but location and/or microclimate can extend that. Look at whatever JM's are available in your area - otherwise, check out some good JM's for colder climates, like in the attached link. Here is a link that might be useful: JM's for a cold climate

Maybe yes, maybe no :-)) Seedling Japanese maples can often show a great deal of variability from the parent tree in terms of habit, leaf color and leaf size. That's why you cannot get named forms (cultivars) from seed. However, a good many times seedlings will resemble the parent tree quite closely. All you can do is wait and see. FWIW, virtually all commercially grown JM's are grafted and the location of the graft (combined with typical habit) is what determines ultimate height. If yours is a weeping form, chances are it will stay relatively short.

I'm in zone 5 Ottawa,Canada. Zone 4 for you in the US. They used to say you couldn't grow J.maples around here. But for 20 years I always could, and have never lost any, except 2 variegated ones. I have about 25 varieties, a sort of J. maple forest in my back yard & I am running out of room. Good thing they are dwarf trees, unlike our native maples that grow to 50 ft. I wholesale them from (grown in)BC, zone 7-8. They do not loose hardiness by grafting and propagating in a milder climate. The hardiness is built into the plant. I will say that they can sometimes be slow to establish, and this is the important part. Any tree should be planted early and well cared for in the first season, so it is well established going into it's first winter. I don't believe in bare root trees doing well, too slow to establish, or over wintering in a pot. They should always be planted in the ground and well mulched. I do think they prefer a more acidic soil. Ours is neutral approx. 6-7ph, so I spread sulphur everywhere every couple of years to acidify a little.(I don't like pink hydrangeas, prefer blue) I saw a study once, out of U of Michigan I think, where when they tested plants, and they found that plants growing in acidic soil seemed a little hardier than the same plants growing in more alkaline soils. In the past I have used geo-textile fleece as a winter cover for the first few winters, but snow makes a great insulator as well. I get a bit of tip dieback sometimes, but it doesn't matter since they grow back very well, as if they were pruned. There is a sort of wilt going around in J. maples lately, others have noticed as well. It doesn't hurt the tree, it just attacks some of the newest growing tips as they are finished flushing out in summer....sort of self pruning. We regularly get to minus 22 celcius (-10 far.)here most winters, with minimal problems. Even with last winter, the worst in 20 yrs., there were no losses. The only damage I ever notice is the heavy snow pack in spring, will sometimes crack the weeping types as it weighs them down and melts. Variegated types do seem less hardy, possibly due to the fact that variegation is a mutation and therefore weaker. Red and green types are perfectly hardy. They all seem hardier as they get older. thanks, Rich