You are asking for general advice in a very specific visa group and providing no info. You'll get better responses if you ask in one of the general Thai visa advice group where you will get more mileage about different visa options. Then you can start your research as to what suits you best.
You need to check with the embassy you apply at. Some, South Korea is one, state that you must remain in country until the visa is issued. In another group a person was denied in Poland after they had already left the country. Embassies can ask for additional proof you are still in the country or even ask you to come in to the embassy for an interview.
Yes, they stamp your passport as usual when you arrive and show your DTV. You would have to leave and re-enter Thailand to activate the DTV.
It actually depends on what embassy you applied at. Some, South Korea being one, actually state that you must remain in the country until your visa is issued. According to other posts, Poland has also denied visas to people that left the country before it was issued. As
There are numerous posts about tax in this group so suggest you do a search. If you stay 180 days you can be considered a tax resident in Thailand, regardless of what visa you are on. Every situation is different depending on tax treaties, amount and numerous other factors that would be better discussed with a tax accountant.
as John and John have mentioned language doesn't show under soft power. 1 person posted about being successful but several posted about being denied. It clearly states what qualifies under soft power so your best idea is to pursue a category that has proven success like Muay Thai with a gym that has a proven track record, several who post in the group regularly. Either that or pay an agent to help you as it really seems you are getting confused and starting to consider a risky option.
Unlikely, education is not listed under the soft power categories of the DTV. Someone had reported they were successful at the London embassy but other have since reported they were denied.