Ok, you're going to get a lot of flack for much of your approach, put your hard hat on....
It's up to you now. Are you tough enough to see through your convictions for this child, despite a wall of opposition 6 feet deep? Because you're right, plenty of people hear 'autism', non-verbal etc and stop having expectations, focusing most on reducing impact on family life, because it can be beyond exhausting. With two young children of your own, you are going to be plenty busy enough with them, let alone advocating for your DP's child.
If your gut feeling is this child is alert, taking in the environment, responsive to you, then non-verbal might not be a permanent state of affairs. I studied with a man who didn't speak until he was 4, and then straight to sentences, and on to a 1st class honours degree.
What is she doing on the iPad? Age appropriate? Any early reading? Has she had any Makaton/signing training? Does she get specialist nursery education? Is there any respite care available - some churches do this. Are you prepared to work with Mum for her? How are the finances, because adequate money would be really handy if you want to go on a crusade for this child!