I'm not to sure about the answers to some of your questions, but I can have a go at a couple:
1. A TAF is pretty much only accurate to the airfield boundary. A nearby TAF would give you some idea about what's happening locally, but I wouldn't take it as gospel. Have a good look at the area forecast as this will tell you about cloud, wind and vis.
3. Ah, valleys. I'm no expert, but some "gotchas" to look out for.
i) Never go into a valley unless you have an "out". Your escape can either be via a turn, a climb or by going up another valley, but don't ever go in one that you are unsure about how you will get out of it.
ii) Think wind! Valleys are places where downdrafts and rotors like to live. If there's turb around or if the area forecast says that winds are strong (above, say, 20kt) then it's a good idea to put your valley flying on hold. Aiming off from a ridgeline by 45 degrees helps give you an idea of wind before you commit.
iii) Vis. When there was low cloud around we always used to check we could see two ridgelines before committing. That is, if we were crossing rows of features at 90 degrees, we'd make sure we could see the one we were about to cross and the one beyond that.
It's been a few years since I've done valley flying. Some old PNG hands should be able to help you out with more detailed info.