If you mean using the walls as support beams for the deck. I don't see a theoretical problem with that as far as support, if they're built correctly. Especially the one, which used to support a porch. Are the tops even with each other? Or will you have to have some additional framing on one of them to bring it up? That could get a bit wonky.

The small tree is a tree FORM. Make it out of a large shrub so it's roots will not be too extensive. What is the brick cut-out? It looks like an unfinished construction. I figured it was temporary and would be "healed." Next step for you is to create a base plan where all the features (house/wall/deck/tree trunks/shed/fence, etc) are located accurately. Then draw bed lines. Begin placing plants. Use appropriately sized circles for larger plants and outlines for plant groupings. If you make multiple copies of the base plan and scratch out your ideas until they are refined. You've got to convert ideas in picture, refining as you go, to a PLAN in order to know that things fit, are properly located and so you can calculate quantities. Once the plan is finished, you can create a plant list based on it. And then go shopping. If you're doing irrigation, it should be installed after large trees (if any) are put in place, and before all the other plants. If no irrigation, Fall is the better time to plant IMO as roots get more time to develop before hot weather arrives.

"...builder screwed up and didn't plan an appropriate exit..." Well I guess. Is the builder willing to rectify the situation? It looks to me that it may be possible to build a deck outside the back door which projects beyond the face of the house, with minimal steps and/or ramp in the direction of the left side of the photo, parallel to the face of the house. In other words, a deck projecting out beyond the house for whatever dimension you want with access/egress toward the left margin of the photo, where the grade appears highest. It appears that the ground slopes off so much to the right side of the house that it's impractical to build anything in that direction. What do you prefer to do?

^ Good idea. It would be nice to see all that cement re-done, if you can afford it. Or maybe just put some pavers or a concrete slab from the doorsill to the wall (it looks like at least one sill is a few inches above the concrete) which would create the first step. The wall itself doesn't look so tall as to need more than one step -- although pics can be deceiving. It just doesn't look like a good area for a wooden deck, unless you particularly want something raised above the ground on posts. A paver/block/stone patio would work and be pretty uncomplicated to install.