I do agree that a marriage proposal is not going to fix their issues and a lot of work needs to be done, however, I also think this is an important step in fixing some of those things.
What the marriage proposal (hopefully) says is that Mitsuki isn't going anywhere. She's not going to retreat or runaway like her Uncle did with Kanna. She's saying to Aya that she wants to be there for her for the long haul. Mitsuki is saying that she wants to work on these issues together. I am hoping that this is a step in that process.
And to be fair to Mitsuki, she often works on her issues by herself and it takes her a while to get to the same place as Aya. So ch 168 was her buying new music and in ch 169 she's listening to music with Aki. She's was making steps towards getting over some of her issues, and Aki going missing kinda pushed her forward several steps in the process. I don't think that solves everything, not at all, but the marriage proposal could also be seen as Mitsuki saying to Aya even if it takes me some time to get back to myself I'll still always be there for you.
Of course, this could just be my wishful thinking. It will depend on the follow up. I'll be kind of annoyed if this is the solution to everything.