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@Dolii The convergence needs to be in $L^2(D)$. This amounts to the boundedness of the the harmonic extension from $L^2(\partial D)$ to $L^2(D)$. I believe this to be true, at least assuming $\partial D$ is smooth enough, but I have yet to find a proof. There is a Springer book The Dirichlet Problem with L2-Boundary Data for Elliptic Linear Equations on that particular problem, which may be helpful to you.
Yes, and your attempt showed how to approximate when the boundary value is 0. Then you only need to approximate the boundary value in $L^2$, while keeping the Laplacian 0 at the boundary, which can be done using the hint.
Take a simple example with maximal degree $d^n$ (for example, $x_1^d=2$ and $x_{i+1}^d=x_i$, $i=1,\dots,n-1$). Then make the change of coordinates $x_i'=x_i+x_n$, $i=1,\dots,n-1$.
Try the simplest example: $R=\mathbb Z$. Then every maximal ideal is of the form $m=(p)$ where $p$ is a prime number. Then $R_m={\mathbb Z}_{(p)}$ is a DVR, so every two ideals are related by containment. On the other hand, there are plenty of pairs of integers $(x,y)$ such that $x\not\mid y$ and $y\not\mid x$.