Well done on your progressions so far. The shaking is most likely due to the fact you’re new to the exercise and it is putting your muscles under more load then they’re used to. My suggestion would be to stick with the incline push ups at the current rep range working on good form and work it every other day if you can. If that is too tough cut down the reps and increase the sets (i.e. move from 1 x 15 to 3 x 5 – 8) and then build back up. There is no need for you to go back to wall push ups.
Come back and let us know how you’re doing in a couple of weeks!