How should I fill channels left in old plaster after dado rail removal?
I don't understand option 2, but option 1 is exactly what I'd do. A few suggestions:


Be sure that the thickness of your drywall brings it to flush or slightly below. Do not install patches that protrude above the wall surface. This will be difficult to tape without leaving a bulge. 
If you end up with a depression after installing the patches, pre-fill before taping, possibly with a setting-type compound (which is more difficult to work with, but cures quickly, is stronger, and doesn't shrink nearly as much). 
Don't worry too much about getting a lot of screws in the drywall patches. The two close tape joints will hold it all very solidly. You don't want to crack up your plaster any more than necessary. 
Use a nice wide knife for your finish coat(s) and taper out substantially. I'd go no less than 12" from the edges of the patch. Skinny repairs will leave noticeable ridges. 


Good luck.