Almost any aircraft is capable of entering a spiral dive (many do, for example, upon recovery from a spin). My understanding is that excessive spiral stability (which implies high lateral stability, or weak directional stability) defines difficulty in recovering from a spiral dive, rather than necessarily a tendency to enter one.
I don't think it's correct that spiral instability implies a tendency to enter a spiral dive, the co-incidence of terminology there is unfortunate. It happens that both spiral stability (defined as the tendency to return to wings level when disturbed, or not), the tendency to spin, the DR ratio, and the tendency to recover from a spiral dive, are all functions of the ratio between lat-dir stabilities. This makes them interdependent.
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