I just did a lot of the trip you are planning last month. I posted a trip report with too many pictures, I'm sure, but it was a lot of fun. You have arches as a requirement and was asking about whether you should do Moab. If you mean Moab UT, that is where Arches is. It's also just 30 miles from Canyonlands, which I loved even better than Arches. But Arches was great, too.
Hwy 12 was discussed and I concur that it is fun and scenic, especially just south of Boulder on the Hogback. Bryce Canyon is also on 12. As for 191, I was on it from Blanding to Moab and it's not that great. And 491 is Devil's Highway, not 191. It was 666 until 2003 I believe.
Redfish will be a big help to you. I planned a lot of my trip from his advice.