Wow you got year old scion to take, even at 10% that sounds like you did good for being that old. Yes the diameters are very important if you want to get the best contact. Even with the correct diameters however, it is difficult to make the cuts perfect ovals that match eachother perfectly exact. But that is ok, just get them pretty close. Also remember to start the tongue just above the center of the oval, I start mine just above the tree core. If you put the tongue in the middle they wont slide together snug at the correct location. Lock your thumbs together when you cut the tongues so you dont cut yourself because that cut goes with the grain of the wood and once it starts it just lets go really easy and down comes your blade right into your hand. It has put many grafters in the hospital so lock those thumbs. I think if you take your time, and use fresh scion, you will have a much better experience this year. Dont let last years attempt scare you, that old wood was probably the main problem. Apples really are pretty easy to graft. Sounds like you mentally have it figured out what needs to be done, im sure you will do great this year.
As far as the root stock goes, i would probably take them out of the bag they are in and stick the roots in some moist peat and put them in the shade somewhere cool, in the garage or something like this. They will be fine. No need to stress over it if your going to be working them soon. Worst that can happen is that they start leafing out, and even then you can still graft on them just fine. I was just field grafting today on some rootstock that are starting to leaf out. I remember stressing over that same issue when i started, most of the grafting information sources dont explain a lot of the details like this and leave you believing that everything must remain perfectly dormant and sterile in some highly controlled lab environment.
Here is some other guy grafting in the field just like i was doing today, notice the rootstock has broke dormancy.