Some thoughts.
It should ne noted that a decline of the PRI, even without a lengthening the structure of production can still lead to speculative bubbles and malinvestment. It only requires that spending to be higher in the stages of production further from consumption, not in the stages closest to consumption.
Scenarios 1, 3, 6 and 7 fit this pattern. But recall that there is no impact on growth in scenario 6 and lower growth in scenario 7. It seems to me that the bubbles grow to enormous proportions, certainly through monetary overexpansion, but also because the economic growth is strong. For “euphoria” to be manifested, everything must increase at the same time, which is possible only if the new money is continually injected into the system (see Fritz Machlup cited in Money, Bank Credit and Economic Cycles, page 462). Without monetary overexpansion, a rise in some prices implies a fall in other prices, all other things being equal. If malinvestments occur, given the fact that scenario 7 implies a less vigorous growth, malinvestments are likely to have less serious impact on the economy (compared to 1929 and 2008, for instance). In addition, scenario 7 does not involve an increased supply of present goods; this poses a theoretical problem because any monetary overexpansion necessarily implies an increase (though illusionary) in the supply of present goods, through a lowering of interest rates. Scenarios 6 and 7 therefore do not contradict the fundamentals of the ABCT.
In scenarios involving an increase of the PRI, we have consistently the same pattern. A lengthening of the structure of production and a thinning of the stages of production further from consumption, the latter necessarily implies that expenditures in stages further from consumption do not increase. In this case, malinvestments cannot arise.
The scenario n°2 shows a curious pattern. The PRI does not drop, but the structure of production lengthens. At first glance, one might think that malinvestment can take place. Given the fact that ABCT hypothesized that “artificially low interest rates through monetary overexpansion will trigger malinvestments” the core of the ABCT seems to be weakened. I think, however, that we can reformulate the theory as follows : “monetary overexpansion will trigger malinvestments” (whether the interest rate drops or remains constant).