I wouldn't want to be without any of them, but if I had to choose between the 28-105 and the 31 I'd go with the 31. Together with the 100 you'll have a fabulous kit.
I prefer using primes when I can, but when I need a zoom the 28-105 does a wonderful job. I find it nice and sharp at all focal lengths and apertures.
The hardest to replace will be the DA15. It's quite unique in being such a tiny UWA, super flare resistant, and wonderfully contrasty. The DFA15-30 looks like a fantastic lens, but it's huge (and expensive). My solution has been to get an A20/2.8. It's quite light and compact, and performs well. Not quite as the DA15, but I enjoy using it. (I have a few shots from the 20 here:
Pentax-A 20/2.8 | Flickr)
I have no experience with the Samyang 14mm, but those who have it seem happy enough.