It's reasonable to continue your case without an active DNI. In fact, I know a guy who got citizenship without ever having residency, and got his DNI on acceptance as a citizen.
As far as showing your mode of living, my friend didn't have a bank account (in fact, still doesn't) and was never paid here in the white. He had three people come before the judge's clerk (I was one fo them) who swore that he was a good guy, that he made a living doing what he did, and so on. It seems more of a formality than anything.
My friend was also advised by his lawyer, who helped him through the process, not to leave the country during the process. As to whether you need a lawyer - I'd say it's not a bad idea, but I know of people who did it totally on their own, although I believe they already had a valid DNI when doing it. For all I know, the fact that my friend who got citizenship without a DNI had no problems specifically because his lawyer filed the correct motions to get things done properly. Anything I've mentioned above is simply me telling you the experience of others, not necessarily the way things will be for you, as things are not always cut and dried and the same for one person to the next.