I solved the problem. First, I noticed that Mint Daryna was complaining about 915 and 810 Intel video conflicts so I uninstalled them in Synaptic, not needed for AMD 690G. These were installed by default.
I finally realized that Envy (or synaptic) was installing older versions of fglrx that evidently limits the resolution to 1280x1024.
I reinstalled the latest ATI driver manually according to the Ubuntu install guide. NEVER use the "Test" button when choosing displays, all tests will show corrupt screens, even though just clicking on OK works. Test wasted a lot of time. After some more testing and choosing monitors I did get it working in 1440x900, but 1680x1050 was not an option. More trial and error with choosing monitors finally got 1680x1050. It seems flaky when selecting monitors and resolutions sometimes, but 1680x1050 was finally there on reboot. There are no errors in the logs now.
New problems, when I tried clicked on Quit in the Mint Daryna menu, the system froze on a black screen. Searching here suggested disabling a daemon, which I have done but not reboooted yet. I also automatically get kicked out of the desktop, back to logon screen, if I try to run glxgears now. Scrolling in Firefox is slow.
I'm sorry, but buying 2 new widescreen monitors and resetting the resolution shouldn't be this difficult. I haven't even pulled the second monitor out of the box to try it yet. This stuff may seem easy or second nature to others who do this a lot, but even though I've used Linux for a long time, I don't usually do a lot with resetting video. It took 4 nights to figure out what was going on and how to get it working. I'm not sure I'm looking forward to instaling the next set of "revised" video drivers.