Re: Backyard Landscaping
In general, I think your stated plan is a decent approach, although you may want to call a landscaper or other such person (soil scientist, or call your local agriculture extension service for likely free advice) to consult with you and offer some advice from real world experience. It may be as simple as tearing up the top 6 - 18 inches of clay topping that with good soil, although I'm not sure where you're located and the exact profile of your local soil.
As for the buried utilities, I'm not sure what you have in the yard besides the gas line, but utilities typically don't move buried structures unless there is a safety issue or other compelling public interest (at least here in PA); although you can probably pay someone to move and re-install for you. Call your state's utility markout service (we have one here in PA [PA One Call] w/ an 800 number) and have them markout the approx. locations in your yard. DO NOT dig if you don't know where the utilities are; in many cases once a utility crosses onto your property, you are responsible for any costs necessary to repair it, particularly if you damage it (acts of God are another thing). Depending on their age, public and private utilities are generally buried at least 18-inches below ground, and, at least in PA, you have to assume that, when marked out, the utilities can be at least 18-inches on either side of the marking line.
Anyway, hope this helps a little bit. Good luck!
-Daryl, learning how to keep up with an 80 year old house in PA