Maybe better to create separate threads in the forum for each issue. Else it is hard to respond.
"Sub task can be converted to millstone, however, millstone can not be converted back to sub task as it show below"
You can set the duration of the milestone again, there is no menu item, convert to normal task.
Concerning the project date, it is read only calculated. The project field in the header is a start date for all loaded projects in the chart. Maybe we have to rename it to 'timespan' date.
"Gantt chart is not linked with Kanban chart, the details is not converted as well to the Kanban chart only the name of task is converted "
We use are own fields for storage, not all fields are mapped to project module fields. We have no detail mapping, if you wish that we make some new mappings I can create a feature request. (Please open a new post for this)
The same thing counts for the actual startdates, we have no mapping here yet.
About the error. Can you reproduce it at our demo site? I need at least a trace log and steps to reproduce. (please open a new thread for this)