For a Samsung NX100 on a tight budget, which upgrade helps most: bounce flash, telephoto zoom, or 30mm prime?

I personally think a flash with bounce and more power offers a lot more flexibility than an additional lens. In many situations you can make up for lack of reach by just moving your body, or fast glass by doing long exposures, but there are just a lot of scenes you can't shoot at all without a diffuse light source (e.g. when fill flash is needed, or an indoor portrait, where a long exposure would have motion blur). Other things you didn't mention:

1. If you don't have a flash diffuser, they're really cheap and would expand your options with both the SEF15A and SEF20A, allowing them to be used for fill flash and possibly macro. However they're weird shapes, so most slip on diffusers won't fit. There's a thread on dpreview about using pop-up flash diffusers for the SEF15A (the Opteka seems to work okay).

2. A tripod opens up more options for long exposures and self-portraits. If you don't have a tripod I'd really recommend a small portable tripod that would be suited to such a small lightweight kit. For example the GorillaPod Micro 800 is 30 USD and the Samsung NX100 with kit lens weighs 463g, well under its 800g weight limit.

3. Some wide open environments don't permit mini tripods because there are no objects around to hold them up. In that case I'd go for a collapsible light full size tripod like the Tamrac TR406, which goes from 11 inches (28 cm) folded to 44 inches (112 cm) deployed, supports 3 lbs (1360g), weighs 11 oz (311 g), and is also pretty cheap (50 USD).

4. A remote or (better) wireless shutter release (e.g. the Pixel RW-221 has an NX100 model). These greatly simplify self-portraits and are very helpful for long exposures on tripods, since they let you shoot without introducing any vibrations that could cause motion blur. They also tend to be quite cheap.