English holly is a pest species in the Pacific Northwest, planting a hedge of fruitful female Ilex aquifolium would not be a service to the neighborhood. Hybrid Highclere holly is nicer looking, does not get leaf disease that may cause lower branches of English holly to defoliate. But it has bigger leaves, less suitable for clipping (the smaller the leaf size the better for clipped hedges). Other traditional hedges are yew and box. Nothing fast-growing will provide comparable density, if you want something as crisp as clipped yew you will have to wait for it to grow up.

The red one is the nymph of the Leaf-footed Bug, usually considered a minor pest of a wide variety of plants. Can't really see enough of the others to tell you for CERTAIN what they might be, but my guess is that they are the adult Leaf-footed Bug.

We can only see a very narrow part of the space from your photo, so it is difficult to be sure what might work. How big is the gap you are trying to cover up? Would you want taller plants during the growing season along with the lower plants to cover the gap? Would a row or two of cobblestones or landscape timbers be enough to cover the gap, such as Mad_gallica suggested? Can you set this up so that one panel is hinged and moveable for access under the deck, allowing you to plant in front of the rest of the panels in the ground? Or will that count against you as disturbance? If you plant native wetland plants that will help filter runoff is that an issue? Can you plant things in the ground that will recover fine from being cut to the ground for inspection? How often are the inspections? For instance, spirea and Hydrangea arborescens such as 'Annabelle' can be cut to the ground and will recover within a few weeks. Neither of these are evergreen, but if that type of planting is an option, I will consider other possibilities. If you are wedded to the idea of these planters, you might try a series of very hardy evergreen perennials and shrubs this year, and let the results next spring tell you what to plant for the final planting based on what survived. You can think about buying better insulated planters, either larger ones and lining them with a couple of layers of small bubble wrap internally and then mulching the soil surface well in autumn, or extruded foam planters, which I have seen at my local big box store. Because the foam holds air, it is light weight and provides insulation, and they resemble pottery. I find clusters of pots more interesting than only a straight line, so can you add some groupings of pots in front of these planters to break up the line? I can't see enough of the area to judge, and those groupings could have some annuals for a longer flowering period. Check out Geranium Biokovo Geranium macrorrhizum Iberis sempervirens/perennial candytuft Vaccinium vitis-idaea/Ligonberry AKA mountain cranberry Dianthus plumarius or D. gratianopolitanus/Cottage pinks Sedum 'Angelina' variegated thyme Veronica 'Georgia Blue' groundcover Junipers such as Blue Star All of these survive for me in the ground, including some that are in exposed settings that freeze pretty deeply. I can't help you with info on soil mixes. I just use a mix of regular good quality potting soil and coarse compost in my containers. Or there is always Sam_MD's solution of reasonably realistic plastic shrub for the winter and flowering annuals for the summer.

Try posting this in the dedicated plant ID forum - Name That Plant - for fast and accurate responses. This one is really for design issues. And be sure to include your location!