, lawyers may discuss forever what an "extract" is? Somehow you process it for smoking. Bottom line is that the law only allows you to grow it - so far. We are awaiting the laws on recreational use. However, it is being sold and smoked in large scale as we speak - legal or not...
, legal to grow it for Thail nationals but illegal to make any kind of "extract" from it without permission, which is currently only available for medical use or for food with very low THC levels. New regulations are in the pipeline without a schedule.
The law says it is legal to grow for Thai nationals. The use of cannabis is by law restricted to medical use and food. Laws about creative use are expected in the near future. Right now the situation is a limbo for creative use. No one seems to care much. Until 1978 cannabis was completely legal in Thailand.
As a general rule, a visa can only get extended once with the same kind of extension. The exception to this rule is the periodic extensions. i.e. annual extensions etc. In this respect a visa exemption is also a "visa"...
Yes, until your extension expires. Then some Immigrations will ask you to do a 90-Day report same time as getting the next extension approved, while others will let your 90 days expire after 90 days...