Not exactly sure what you are trying to accomplish, especially the prior observation field, etc.
However, in case it might help, I have tested this arcade with the editable logic:
var arr = ["Observation 1", "Observation 2"] // List to compare to
if (Includes(arr,$feature.ObsvPeriod)){
return true
}else{
return false
}
If I have a field that is calculated (and thus not editable) the calculation will fire as normal. However, if I attach the above code to the Editable logic expression for that field, then, if the values of "ObsvPeriod" are equal to "Observation 1" or "Observation 2", then the field will become editable. If it is any other value, the field will be populated by the calculated expression attached to it.
In case this helps somehow,
R_
edit: I see @AlixVezina beat me to it....