Do, spread the roots out and stick it in the ground. Check this cherry out; 2 seasons in the ground (I'd have left it in the ground if we weren't moving). The base above the nebari was 1.5" in spring '08, and now it is 5". The heavy roots are still pretty coarse, but each has a smaller replacement nearby, and next year, I'll work them back again and stick it back in the ground for another 2 years...
Black cherry is very frustrating and not easy as bonsai. I've had one for 10 years now and I can say it is very very frustrating to work with.
This is a "pioneer" species. it is quick to grow in areas that have been burned, excavated or otherwise disturbed--its niche in the forest is to quickly establish itself by any means necessary to outgrow slower growing species). As such, it has quirks that frustrate bonsai training. It is very, Very, VERY touchy about top pruning. All hard runing must be done before the tree exits dormancy. If you prune anything much after leaves have emerged you get die back down the stem--sometime all the way down a branch.
Additionally, branches you've worked a long time (even five year-old or older, branches) to build can suddenly die off. There is no hard and fast reason for such die offs, just happens with quick growing species like this one (and a few others). So if you plan to spend time developing the sapling you have expect big redesigns due to branch loss along the way. It's just part of the territory.
Leaves do reduce, but not if the plant is happy. To much water and fertilizer and the tree will push regular sized, or bigger leaves, eagerly. Use a freer draining mix a full sun and you may get some reduction.
And no, flowers and fruit do NOT reduce on any species--Flowers on this species aren't the nice neat single buds that people expect. The buds and "fruit' (they're more like berries) are born on spikes that have dozens of flowers--
The blooms are awkward to work with and can't gracefully be incorporated into a bonsai design (the smaller the tree, the worse they look)...I usually cut them off. If you leave them on and allow the plant to produce fruit--beware of birds (they love it)--smaller trees can be snapped in half under avian assaults
Yours doesn't look like Serotina--which is very different from the Japanese cherries.
Prunus serotina is a very tough plant and can make good bonsai--IF--and it's a big if--you are willing to put up with it's weird habits. Most of the serotina around here have boring smooth gunmetal grey bark and uninteresting trunks. I was lucky to find one that had old flaky pine-like bark and some movement and character in the trunk. That interesting trunk is the only reason I have one of these.
Oh, and if the unstable growth habits don't get you, the bugs will . Mine, like all serotina around, are magnets for tent caterpillars or bagworms (whatever you call them). The little beggars completely defoliate big cherries in the woods here every year. I pick literally dozens off the tree every week during May and even then I lose significant amounts of foliage off the tree by the end of "caterpillar season."