I always wonder what people mean when they say "on a good day". Probably more useful for your trip would be - what do you feel safe and confident on when you rock up to a new crag? Considering you are turning up at 2 new crags with only a few days each - it can easily take that to get into the groove of a crag and I wouldn't expect to be doing what I can do on familiar crags for longer than that.
What did you end up doing on your flying trip to Buff? That will probably give you an idea for the Valley, but you are right, it has a lot more easy options than Moab. I haven't done anything outside of cracky heaven there, but I remember very little below 5.10. Although the classic 5.10s like incredible handcrack tended to be cupped hand cracks for me, so if you have normal to slightly large hands and are confident lobbing them in, they should be cruisy enough. Have you climbed at Frog or Ben Lomond? Or done any of the Blue Mtns splitters like Kaladan or Interstate 31? Lift girls Lament or Paralax at Buffalo? That would be as close a style as I could think of in Oz for grade guestimations. Just take every single face feature out of the equation! Have you thought about going via Red Rocks instead where there are lots of big easy routes plus sport climbing? Or doing as Mark suggests and hiring a guide - often the best way to suss out any new area when you are on such a tight schedule.
My thoughts timewise would actually be just to spend the whole time in the Valley. 10 days isn't really a lot and you certainly won't run out of things to do and you will probably end up leaving just as you start to get used to the climbing if you try to do both. Weatherwise, I'd swear it would be too cold, but my advice on temperature always needs to be taken with the knowledge that I seem to experience weather about 5-10 degrees differently to most people. Actually, it was pretty warm in the Creek at the start of October when I was there and 1 week after I left it was snowing, so it could do anything.