Yes, I do think Derek believes that Meredith will have Alzhimers like Ellis. He told her what he would do if she ended up having Alzhimers like Ellis.
Derek: Ready to go?
Meredith: I just got a page. John Doe needs to go back into surgery.
Derek: Yeah, his post of CT's showed a re-bleed. I'm gonna have Nelson take care of it.
Meredith: No. Really?
Derek: Yeah, well I'd prefer to do it myself but, I wanna check in on Izzie. Maybe we can do that first, and then go get married after?
Meredith: No, we can't because we have the George intervention after that.
Derek: I'll have Nelson do it.
Meredith: You know what, tell them to prep him. We'll run to city hall. We'll come back, and we'll do it then.
Derek: Meredith.
Meredith: No, you know lets go. We gotta go. We gotta run to city hall, we'll come back, you'll check on Izzie, we'll monitor John Doe. I'll go talk George out of joining the army.
Derek: Look, we could do this another day.
Meredith: There is no other day! Every day is like this! Every day there's a crisis! There's no time!
Derek: Meredith.
Meredith: I love you and I do want to marry you today. But, there is no time.
Derek: (Der puts his hands on her face and looks at her) Do you have a piece of paper?
Meredith: For what?
Derek: I wanna be with you forever, and you wanna be with me forever. In order to do that we need to make vows. A commitment. A contract. Give me a piece of paper.
Meredith: I don't! I... I... I don't. I have post its!
Derek: (takes the post its and a pen) Ok. What do we wanna promise each other?
Meredith: That you'll love me... even when you hate me.
Derek: To love each other, even when we hate each other. (writes it down) No running. Ever! (Mer smiles and comes and sits in front of him) Nobody walks out. No matter what happens.
Meredith: No running.
Derek: (writes it down) What else?
Meredith: That we'll take care of each other, even when we're old, and smelly, and senile. And... if I get Alzheimer's and forget you...
Derek: I will remind you who I am, every day. To take care when old, senile, and smelly (writes it down) This is forever. (signs it) Sign.
Meredith: This is our wedding. A post it? (smiles)
Derek: Mhmmm. If you sign it.
Meredith: (signs it, and hands it back. Has tears in her eyes) Now what?
Derek: Now I kiss the bride. (They kiss)
Meredith: (smiling) Married.
Derek: (smiling) Married. (sticks their post it in Mer's locker) You see that? (Mer nods and smiles) Plenty of time.
He told her that he would support her no matter what.