Your post asks many questions, and even more questions arise in trying to formulate answers to the questions asked. I think you might get more response if you focus on one essential thing, and also if you can post a photo or two, it will help bring the questions into focus.

The best way to improve soil in a bed generally is to add organic material -- i.e. compost of some kind -- not to dig up soil "from the lakes and creeks" you "live near." That, in fact, would be highly ecologically irresponsible and you say that you want to avoid that. Whether you need to mortar the rocks in a wall depends on how high the wall is. By the way, building a stone retaining wall more than a few stones high requires some engineering and you'll need to read up on that before you take in on as a dyi project. There is no magic cure-all for weeds. You need to pull them -- all of them -- and then try to minimize (notice I don't say eliminate) regrowth. The best way to minimize regrowth is to mulch with a few inches of organic mulch. You can use several layers of newspaper under the mulch (around the plants) to further impede initial weed regrowth. Wood or bark mulch is fine if you can afford it. If not, there's nothing wrong with chopped up leaves. Avoid stone mulches as they are pesky to maintain over time. Even with a good mulch, however, you'll still get some weeds -- it is unavoidable. Pull the weeds regularly to avoid a big mess again. Before you take on any of this, I recommend you do some more research. It's always a good idea to know what the plants are and what their cultural needs are before you do much planting design or moving around. You'll have better results if you do some homework.

>This garden is constantly overgrown with weeds and I need a cure for that. I'm on a tight budget and really would like to use resources that are good for the environment.

I can solve your problems all at one go.

If your garden is full of different kinds of weeds, the soil can't be that bad. Call your local tree trimmer companies and ask for a load of arborist wood chips. They generally will deliver for free, though on their own schedule. Pull all existing weeds and compost them. Mulch with a good 4" of wood chips. No more weeds, ideally earth-friendly, and as the wood chips break down, you have garden soil of perfect loveliness. Supplement with rich compost from your pile (top-dress it!), and you'll have garden soil the envy of the neighborhood.