Actually, I rather liked the graininess. This forum post
includes a technique in PS for reducing noise without the need for a plug-in.
One of the problems in your images is the relative contrast between the occasional areas of sky (highlights) and the darker tones. You can try changing the metering to centre-weighted to retain the people (and make the surroundings even eerier). Alternatively, use reflectors, which are bulky and not very effective in low light. Other than the techniques you used, I'm afraid I'd go for a flash unit, but you can use a "soft box" attachment on it if it's external or gum a cigarette paper in front if it's a pop-up (my favourite low-tech tip). In the latter case, there's little point, but you can use flash exposure compensation to attenuate the harsh effect.
If you do use flash, keep the ISO setting as low as possible - no point making a bad situation worse.