The docking of the preview window in P12 is completely off. In P11.3 that window docked as a full panel, not constrained as you describe it. I don't know if anyone has reported that problem and whether it's on its way to being corrected. I might go ahead and report it anyway. Your best workaround for this problem is to click on one of the buttons on the UI Dots to save the workspace as you want it. As for your second problem, I suggest that you open the recent renders panel and look at what is being saved as cached files. Clicking on any of those thumbnail images will appear in your render window. Then you can export your render.