Heres some answers to these old but still relevant questions:
Overhead of continous thread analysis is minimal and the OneAgent itself tracks overhead and will automatically adjust capture rate of thread samples.
Data retention is the same as Distributed Trace Data (https://www.dynatrace.com/support/help/shortlink/data-retention#purepath).
There is no way in Dynatrace to trigger a thread dump, also no need for it since with continous thread analysis the OneAgent captures thread dumps all the time automatically.
There are a few requirements to get the continuous thread analysis benefits. It is not applicable to all environments. So whether Dynatrace will have to change the minimum requirements or they will have to introduce the manual way.