These don't appreciate shade and would be liable to appear even more gloomy and scruffy stuck beneath another tree than they do anyway. Then there is the matter of the visual tension produced by having a conical conifer stabbing up through a deciduous tree (unless the shade is heavy enough to prevent it from growing up through the branches of the walnut much). If you don't require tall plants use broadleaved evergreen shrubs with rounded or spreading growth habits instead of coniferous evergreens. If you require tall plants that will be likely to grow well above the bottom level of walnut branches it would be greatly preferable to have these located behind or beyond the spread of the tree branches.

There is at least one cultivar of Avocado that is more dwarf, but the species in general doesn't lend itself to container growing long term. At the very least, it will prefer something as large as an oak wine barrel for size, do you have that kind of room on your deck? It will grow for several years in a 15 gallon sized container, but is unlikely to give you any fruit restricted to a small pot like that.

I believe that the link below is for a plant that will survive your conditions. Note that the plant will die down once flowering is done but will regrow from creeping rootstocks. I have some planted in a pile of soil, concrete pieces, and branches that I dumped on top of a driveway. You would not know it. Can provide photo if you wish to see. Let me know. From what I have seen of understory in pine forests or under pine trees (in California) , it tends to be rather sparse or just pine needles. You may need to provide some water when the new growths begin to emerge but it depends on your weather. You can tell if it wilts back that it needs it. Here is a link that might be useful: Ceratostigma plumbaginoides

I have a 60 year old Autumn Glory Maple on the west point of my house. It affords little light to anything below except for winter months and early spring. During that time, many flowering plants will do quite well. I have snow drops, crocuses, squills and daffodils of many varieties. If you have an old grill. Remove the guts, plug up all but one hole and grow hanging Sedum. Forsythia will work very well and only has to be trimmed once a year. The one in the picture is 40 years old and is in dense shade from April to October. Bleeding Heart and Epimedium. Elephant Ear but I have to dig up the bulbs for the winter. I have three potted Clivia plants that give orange blooms in July or so. They too come in for the winter. Variegated Hosta is a cheap standby that can't be killed. Don't worry about dividing. It doesn't need it unless you want an easy plant to propagate.