A few more ideas from another mostly at home mama (with 3 kids), interested in growing food, and so on. These are a lot of things that I have gradually done that might be valuable for you to think about - I don't mean to suggest a person could instantly do them all at once.
1) Life with one car is very, very, good. I think you said that you are uncomfortable biking with the baby, and with no idea what your roads are like, I would challenge you to keep thinking about it. The time of day you bike is often key - at rush hour, no way, but around lunch time, or 3 pm, things are often very quiet and the road is much more comfortable. It is also important to look at actual safety statistics for your area - the places kids are most likely to get injured are the kitchen and the bathroom in your own home. Seemingly safe toys like the super popular two wheel scooters are responsible for more emergency visits than biking in young kids. I'm just saying, do a little more research before you write it off. Especially if you can borrow or get a trailer used (with a big flag), and you can practice a few times at the right time of day without the baby, you may find that it's fun and you feel more comfortable than you thought.
2) Can your husband negotiate working 4 ten hour days, and/or working one or two days from home? Can he set his own hours, and show up really early or late, to avoid rush hour traffic? This could potentially cut commuting costs and time a lot. Then, if he is home one day, it is easy for all the errands to get done on that day with the one car, and easier for you to get part time work, like the 1 day a week option somebody suggested.
3) On packing lunches, you could try a couple of things. One, it sounds like your husband is worried about feeling deprived, so maybe you could get a little inspiration and start packing amazing lunches. There are entire blogs where people share the healthy, beautiful, and delicious lunches they pack for their families. If you can pack better food than he can buy, then maybe it will be easier for him. This goes for meeting friends, too, meet near the Chipotle, not at it, then you can eat what you bring. One of my favorite books is The Vegetarian Lunchbox - even if you're not vegetarian it has a lot of good ideas. Think of great ways to use leftovers - making chicken tacos in wax paper pouches that can be heated up (pack salsa in a little container) and so on. You can also always ask for a trial month - ask him if he's willing to eat what you pack for one month, and put all the money he would have spent in a jar to save. Ask him what he likes to eat at lunch and then see how you can make it happen. This might help him make the leap.
4) With gardening, I would definitely recommend starting with mesclun, lettuce, chard, etc, as they are easy to grow, and the fancy greens will easily recoup your costs. Green beans, too. These things are also easy to preserve - blanch and freeze. With greens, you can plant a row every two weeks now until the middle of June, and then the Middle of August to the middle of september, and eat your own greens at least six months out of the year. If you get really into it, check out the Four Season Harvest by Eliot Coleman. Actually, all his books are amazing and inspiring.
The other thing to consider is joining a CSA (community supported agriculture), especially if you can find one with an option to work. I know there are a lot in that area. This would get you affordable, local, organic vegetables, a lot of experience observing and gardening on someoneelses farm, and a chance to get used to cooking seasonally and preserving a little bit. Part of the skill of growing your own food is cooking it and putting it by, so since there seems to be a limit to how much you can grow right now, you can still get really good at those other skills. (Learn 12 ways to cook chard, for example).
5)Check out pick-your-own places for in season fruit. Every fall I go pick about 20 pounds of organic blueberries, freeze, and dry them. The same can be done with strawberries, apples, etc.
6)Check out ethnic stores - I have found they are the best place to get things like rice, beans, spices, nuts, coconut milk, and sometimes frozen meat, as well. My experience with Trader Joe's is that some staples are cheaper, but it's mostly the packaged stuff. You can always eat more cheaply by only buying staples and making things yourself. Dry cereal, for example, is a huge waste of money.
6) If you don't already know, learn to mend and make minor alterations to clothes. Being able to sew on buttons, repair hems, and so on can save a lot of money. Especially with a lot of kids. I just found a blog
www.makeitloveit.com, where the author is a master repurposer, and I was so inspired by what she could do with her kids clothes and an ordinary sewing machine. We found that people were constantly giving up baby clothes they were growing out of, so it's a great way to practice - for example if there's a long sleeved shirt that fits your baby but it's summer, in about ten minutes one can chop off the sleeves, hem them nicely, and it's perfect. If there's a stain on a shirt, one can cut out a cute little design, sew it down, and in 20 minutes have a perfectly good shirt. If you don't sew, machines and things are pretty easy to find on craigslist. My husband always has to dress professionally, and when I work I do to, so we buy nice clothes, but being able mend and remove stains (and hanging them to dry) extends the life of the clothes a lot.
In a place like DC, there are also a lot of wealthy people with very few kids, so I bet there are some amazing consignment stores and Goodwill - there is no reason to buy clothes new when living near rich people who wear things once and give it away. Even in my poor town I regularly find like-new high end clothes.
I know I have a lot of food ideas, but honestly, I've been feeding a family of five very well on less than $500 a month (including diapers and alcohol) in Italy, where food costs are much higher (e.g. a liter of milk costs the same as a gallon in America, chicken costs as much as US steak), so I am always really am confused at how much people spend on groceries. I think you could set a goal of $250 a month, and then take five months to get there (cutting $50 a month).
This is probably long enough =). Let us know in a few months what you've figured out, so we can be inspired in turn!