Boundaries, Martin's Axiom, and (P)-properties in dual Banach spaces

Let \(X\) be a~Banach space and \(\mathcal{S} \mathit{eq}(X^{**})\) (resp., \(X_{\aleph_0}\)) the subset of elements \(\psi\in X^{**}\) such that there exists a~sequence \((x_n)_{n\geq 1}\subset X\) such that \(x_n\to \psi\) in the \(w^*\)-topology of \(X^{**}\) (resp., there exists a~separable subspace \(Y\subset X\) such that \(\psi\in \smash{{\overline{Y}^{w^*}}}\)). Then: (i) if \(\operatorname{Dens}(X)\geq \aleph _1\), the property \(X^{**}=X_{\aleph _0}\) (resp., \(X^{**}=\mathcal{S}\mathit{eq}(X^{**})\)) is \(\aleph _1\)-determined, i.e., \(X\)~has this property iff \(Y\) has, for every subspace \(Y\subset X\) with \(\operatorname{Dens}(Y)=\aleph _1\); (ii) if \(X^{**}=X _{\aleph _0}\), \( (B(X^{**}),w^*)\) has countable tightness; (iii) under the Martin's axiom \(\mathit{MA} (\omega _1)\) we have \(X^{**}=\mathcal{S}\mathit{eq}(X^{**})\) iff \((B(X^*),w^*)\) has countable tightness and \(\\overline {\text {co}}(B)=\overline {\text {co}} ^{w^*}(K)\) for every subspace \(Y\subset X\), every \(w^*\)-compact subset \(K\) of \(Y^*\), and every boundary \(B\subset K\)