The terrain mistake is indeed mine, not a compiling error. I was fiddling with that part of the map to make jumping off wall more dangerous. Obviously on one of the tweaks I didn’t select that brush and never noticed. I have had errors like yours though in the Helmsdeep terrain, and my only solution was to track down the offending brush and tweak the vertices a bit. It’s not like they weren’t aligned to grid (the entire terrain is locked to 16 or 32 grid, not sure which). It fixed it but was a bit of a suprise, and it can often be hard to track down the exact brush anyway. FWIW I’m still getting texture alignment mistakes on the terrain in places (terrain not blending properly) even though I know the alphamap is set up correctly and all textures match in size. I hate terrain, in other words 
As for radiosity, I’m not a big fan really. The extra compile times are usually just a lazy mapper’s substitute for proper lighting. I like to work without it generally (it’s much quicker - HD compiles in about half an hour full bsp, vis lighting) and tweak lighting manually. It’s easy enough to fake radiosity where you want to, and generally much quicker.
Bear in mind with win XP and only 512megs of ram, it quickly slows to a crawl as Mr Page File kicks in. So it’s not just 8 times longer, but virtually exponential time - I don’t think I’ve let run a full bounce compile since pre FP version 
Oddly enough, despite the nice new lighting stuff, I seem to be going backwards - I don’t do bounce, and Minas Tirith will (by necessity) use a lot of vertex lighting and extremely low quality lightmaps
Couldn’t you get the lightsources and textures to bounce the light?
As above - it’s just a great deal quicker to chuck in a few normal falloffs with extremely low light and fade values (in other words a ‘uniform’ shadow casting ambient component - ‘in other words’ being mine only since it’s a hack but it does the job - nothing I hate more in maps than halos of lighting from non-existant sources!).