Ouachita Thornless Blackberry Plant

They'll sucker, since that is how blackberry plants replace old growth with new growth. However, Ouachita has an upright growth habit, so its canes are less likely to lean down to the ground. This means they're not as likely to take root and spread that way. Any unwanted suckering can be easily mowed down or pruned out.

Yes, they will trellis, BUT, consider this: You will get several primocanes with each plant. If you are careful, you can bend the canes to your will and trellis them. However, recognize that this year's primocanes become next years floricanes, and produce berries. These canes get very strong and woody (to support the berry weight!). Sometime after berry production is complete and late fall, you will need to cut back the floricanes to allow the new primocanes, that have been growing all year, to have room. Un-weaving the woody floricanes and not damaging the promocanes in a trellis may be a tad difficult. Recommend the traditional wire system with soft ties to keep the canes "trained"!

A jumbo cell pack is a bigger version of the cell packs that you might have bought with vegetables at the garden center. These plants are new starts and will grow rapidly during the next growing season.

Maybe a little less than a foot.

Mine grow so tall they basically can fall over, unless I'm pretty aggressive about tipping them -- even then they tend not to branch much and just continue growing from the tip. So I loosely "enclose" them with a wire or two on each side, which really just gives them something to grow up between and then flop over the top of. I'm not sure if it's the Ouachita or one of the other similar varieties I have planted side by side, but one of them does have notably thinner canes which could fall over more easily. But all of mine at least start growing "up" first; if yours isn't at all, I don't have a great answer, just the generic suggestion it could be soil, shallow rooting, insufficient growth requirements, watering, etc.

You don't have to, but the extra support can be helpful for pruning and harvesting chores.

Berry plants are a bit more tolerant of a little shade than fruit trees. The location would need a good 4 hours of full sun a day to produce a decent crop.

You should not have to replace the plants. Blackberries tend to be long-lived. Things that can affect new shoot growth may be too much water or less than ideal growing conditions. Since you stated that you had a heavy crop, it sounds as if you had good growing conditions. I am assuming that your berry crop is done for the year. If not, as soon as it is done, cut out the canes that bore fruit and give the plants a little nitrogen fertilizer. This should kick the vegetative growth into gear. Also, be on the lookout for small critters like rabbits that may be eating the new growth. You may just have a critter problem and not a plant problem. - Agent Ed (Ohio Extension)

In North Carolina and Georgia they fruit through the spring and summer but not into the fall months.

"Invasive" is rather a technical term used by state agricultural departments to describe plants that should be destroyed because they eventually dominate the surrounding enviroment (like kudzu). This plant is not "invasive" in that sense at all. Suckers are Mother Nature's way of propagating the plant for its survival. Many people consider the suckers to be free plants, which they just dig up and plant to create another cane set. If you don't want them, you can snip new growth off just below the surface or dig them up and discard them.