= Solution
For a periodic uncoupled lattice, $\operatorname{tr}M=2\cos\mu$ in the Twiss representation. Here $\cos\mu=1-L_1L_2q^2/2$, so bounded nondegenerate cell motion requires
$$
\boxed{0<L_1L_2q^2<4,\qquad |\!\int G\,ds|<\frac{2B\rho}{\sqrt{L_1L_2}}.}
$$
The zero-strength endpoint is only marginal and usually gives a drifting <Jordan block>. The phase advance increases from zero to $\pi$ on the positive-strength stable branch. Repeating the cell $N$ times gives ring <Eigenvalue>[eigenvalues] $e^{\pm iN\mu}$ and tune $N\mu/(2\pi)$; it does not enlarge the stable strength interval. Integer ring phase advances can be resonance-sensitive in the presence of errors even when the ideal repeated matrix is bounded.
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