Meg, your wall hanging is a beautiful piece of work, no matter what the color.
In regards to your questions, you don't need to re-oil the leather to "seal" the color. A light coat of oil lubricates the leather fibers, but does very little to color (other than darken it). You might consider switching from oil to Lexol, which conditions as well or better than oil but darkens less.
Regarding cutting alcohol-based dyes with denatured alcohol, I believe you can mix it any way you want. It's all alcohol one way or the other -- the liquid is simply the suspension mechanism for the dye pigment.
As for another resist with a matte finish, you might want to take another look at the acrylic-based Satin Shene. Put several very light coats on with a sponge or cut 50% and apply with an airbrush. Either way should give you a nice satin finish.
Although you didn't ask this question, I'd take a look at Tandy's Eco-Flo line of dyes. They're water-based, which means they're not the best choice for outdoor gear that's exposed to lots of moisture, but they're plenty durable and color-fast. And I've found the right custom color is easier to achieve, maintain, and reproduce time and time again. I gave up on spirit-based dyes because every color I tried to create ultimately ended up black or very dark brown. (Which seems to be your experience, as well.)
One more tip -- don't experiment with dyes, conditioners and finishes on a finished product. That's what scrap's for!
Good luck, and I hope the above information helps a bit! -Alex