There are two approaches to leveling grade: one is to "cut" and one is to "fill." Usually, one tries to combine and balance these approaches so that cut and fill are moderated, and so one does not need to "import" or "export" large amounts of material, which could become very expensive. Pushing an amount of soil around a site is cheaper than buying more and bringing it in, or trucking an excess out in order to get rid of it.

It depends entirely on how much cutting or filling is to be done in order to guess the likelihood that established trees will tolerate it and survive. If the amount will be drastic, then the trees will not likely survive and should be removed as part of the project. When it comes to mature tree removal, often regulations and permitting come into play, so it would be necessary to find out up front if the project is going to have this complication.

In the pictures we see a patch of ground, but nothing of the grade that surrounds it, so it's going to be hard to advise you on what scheme you should work toward or what pitfalls you might encounter.

"... our property is unusable. I'm also hoping to put in a new swing-set for our 4 & 2 year old, but will have trouble finding enough level space."

I would like to challenge your thinking on this. To me, a hill, a stream and some trees and shrubs sound like the basis of a potentially extraordinary play area! Consider scrapping the idea of installing a ready-made play set and building instead, a custom creation that takes advantage of the conditions that already exist. Could there be a slide going down the hill? ... steps going up it? ... a bridge across the stream? ... swings from the trees? ... a "fort," ... zip line, etc.? All of these things could be done much easier and less expensively than a thorough regrading/tree removal project, which would be messy and not fun (except for the fact of seeing something accomplished.) These would be fun things to build, too, doing one part of the project at a time. Of course, all of the planning would be done up front and committed to paper so that you understood that all things will fit, work and be do-able. It would begin with a piece of paper that shows the major elements that exist and will be retained, and the property line.

I'm always going to plug finding a local landscape designer, but especially here because we just did a similar project. Every contractor who came in was advocating 8 ft walls and craziness, and we did it with two 30" walls and actually created a stronger transition to get their kid into the wooded area to play. Search for a local pro, there's a button for that above. Even if you DIY it, it'll give you some guidance.

To your questions:

1- you'll have a lot more options if you lose the pines, unless you're willing to bring the wall well in front of them. In that case, I don't know how much value you get from the process.

2- it's worth checking with your city or county. There may be specific regs in your area that deal w/ disturbance of soil within XXX feet of a waterway. It may be as simple as you being required to but up silt fence, but cheaper to know in advance.

3- that decision would be site specific (for example, prevailing winds) as well as specific to the type and construction of wall.

4- I've seen plenty of DIY walls. Some are pro quality, some are horrifying, some are in between. Once you control for the variable of relevant experience, it seems to come down to patience, attention to detail, willingness to seek advice, and willingness to see something that went wonky and tear it out and redo.

If you've done a brick patio you'll have some of the basic skills for wall building under your belt. You can square up corners, you can level an area, etc. Grading just comes down to only using the biggest tool you can confidently use. I have guys who work for me who can use a bobcat to feather out a nearly perfect finished grade that only needs a bare minimum amount of raking. I'd end up taking out a car-sized chunk of hill, so I need to do it by hand (which is why I'm management).

We're in VA so we usethis typical details packet as a starting reference for walls that don't require engineering. Note that if you were to use a segmental retaining wall (as in the interlocking concrete blocks by companies like Techo-Bloc and New Line), most municipalities don't have typical details as in the above. If your wall will require a permit and you use a segmental wall product, you'll need to get an engineer's stamp.

You have an interesting project, and I think bringing in a design pro would be hugely helpful. Yard brings up a great point in his comment, that you have a terrific opportunity to create a blend between more structured space and the woods beyond. I'd love to see that developed, for sure.

I am not sure if you are still looking to put in a play structure of some kind, but we work with slopes and/or trees....no need to do any work on your current yard. Here are a few pictures of sets we have installed in unique spaces.

Tree House · More Info

Tree House · More Info

Tree House · More Info

Just FYI....have a great day!