Lots of people advise to switch straight to raw. I didn't when I made the switch about 4 years ago but think I would if I was starting now.
For making a gradual change it's best to feed 1 meal of kibble and 1 of raw. Mixing the 2 together may cause problems because kibble stays in the digestive system much longer than raw does.
If you feed one meal of kibble and one meal of raw per day, feed half of Maisie's current daily kibble allowance as one meal, then raw for the second meal. The amount of raw to feed is potentially the tricky bit at the moment because Maisie is still a puppy but is approaching the age where you could change her to an adult quantity of raw. I notice the Nutriment website recommends feeding 4-6% of ideal bodyweight per day to puppies and gradually reducing to between 2-3% at maturity.
If you think Maisie is pretty much up to adult size and weight (and is neither skinny or fat at the moment) you could try 4% of her weight for a full day of raw, so 2% for the transition period when she's on half kibble/half raw, which is 260g. Or if you're comfortable to go straight to an adult amount try 1.5%, which is 200g for half a day's rations.
Hope that helps
One other thought, as you said she still scratches her mouth a lot. Is that the only area that seems itchy? If so, have you checked the area for any signs of lip fold pyoderma (infection). If she has it you'd probably notice a horrible smell too. It may help anyway to gently bathe the area and make sure the hair is kept short too.
