I have the DA 25mm myself and it's a great lens. Very expensive but I wouldn't give it up for landscape and architecture work. Your first sentence said, "So I really need a lens wider than the 28mm eq. of the 35A that I currently have." If that's true, there is only one lens option right now -- the DA 25mm. You have a processing option via stitching, but you may or may not want to put up with that. If no to stitching, then the only two questions are can you afford the 25mm, and how long are you prepared to wait for the zoom?
For myself, I never make plans based on vaporware unless some kind of credible info indicates that a new product is truly imminent (i.e. within a handful of weeks). None of us are getting any younger, and I generally like to spend my time making work, not playing what-if with the vendors.

The 645D lens roadmap is very non-specific, and Pentax / Ricoh have been somewhat late to market with the few lenses that have come from it. There's no real reason to believe the additional lenses on the roadmap have been shelved, but also no reason to believe they're coming at any particular time. If it was me, I'd get the tools available now that let me do my work now, and worry about whatever new stuff happens later, whenever it finally comes...