For $k \geq 2, n \geq 1$ let $$M^{n,k} = \{(x_1,\dots,x_k) \in S^n \times \dots \times S^n \ | \ x_1 + \dots + x_k = 0\}$$ This is a compact CW-complex and almost, but not quite, a manifold. I generally want to understand this space better. More concretely, I am interested in the following questions:

(1) What is the dimension of $M^{n,k}$ in a reasonable sense?

(2) For which $n,k$ is $M^{n,k}$ simply connected?

(3) For $k = p$ prime, the space admits a free $\mathbb Z/p$ action by cyclically permuting the coordinates. What can be said about the cohomology of $(\mathbb Z/p)\backslash M^{n.k}$?

For small $k$ and $n$, it is possible to give a "hands on" description of $M^{n,k}$: $M^{n,2}$ is obviously homeomorphic to $S^n$. It is easy to see that $M^{n,3}$ is homeomorphic to the unit sphere bundle of the tangent bundle of $S^n$. The space $M^{1,4}$ is homeomorphic to $S^1 \times X$ where $X$ is the 1-dimensional CW-complex with three 0-cells and six 1-cells, such that for any pair of two different 0-cells there are two 1-cells joining them. $M^{2,4}$ is already pretty hard to describe, but it is definitely of dimension 5.