Well I'm no expert in this put I will put my thoughts into this. For the DNG pictures, that is something you should never use if you are not thinking of editing the picture in photohop/lightroom etc. The only time I use DNG if i'm going to photoshop the picture and get greater details out of it. DNG without editing usually has more noise/grain in them than JPEG when they are not edited so stick to the other format .Having the DNG RAW format is awesome and can give you amazing images if you give yourself time to edit them.
1) Depends if it's dark I usually go lazy and just use the flash and I have gotten great results just with that. otherwise if you want to play with the iso I would recomend that you don't go over 100, the higher the iso is the more grainy/noise it will get. But still you can always go higher but that is just my personal opinion.
2) Same as before Iso 100/200 works usually just fine. Usually in the evening I don't even play with the iso and just have it on auto. Just play with the manual focus instead .
3) Now this is the tricky part so you are saying complete "low light" so I am assuming night shots? First don't ever use flash if you are outside in the dark and trying to take pictures . As for the Lumia930 the auto iso settings are very bad and very wrong ,it goes as high as iso1600 which is abs crazy(hopefully it will get fixed with Denim). The good news is that we have manual ctrl. So it all comes down to the what iso you are using and shutter speed.
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As for this picture which is nothing amazing about but I can tell that this was iso 100 and shutter 2s. My advice is just to start with iso-100 and play with the shutter speed like 0.5 - 2sec(hold still!!) if you feel it's to dark just move the iso up but it will lower the quality of the image. Low light shots can be tricky not only with smartphones but also with Dslr . Just go out and take alot of pictures and practice as soon you get a hang of it ,you will be able to take great low light shots! Good luck!