There is not enough quarantine space for "everyone" but rather only a tiny fraction of the people who want to come to Thailand, so they have to prioritise somehow. It makes sense that one of the groups they are allowing in are tourists with plenty of money to spend to help the economy.
this is specifically only for the national or regional director of companies with a foreign business operation license to operate in thailand with majority foreign ownership. there is a long process to be considered by a committee for the exemption to the normal thai majority shareholding, and mostly only well known multi-national corporations are approved. also the right to work without a work permit only applies to the duties of a director; which would not include spinning decks or whatever a dj does these days.
assuming you already have a work permit, you could be employed as a DJ by an agency and just add that to your work permit. if you don't have a work permit, you would need to register a company (and employ 4 thais) or find an agency willing to pay you at least 50,000 baht/month (i have no idea if that's realistic for a part-time DJ).
you can study and you can own equity in a business but you can't work within that business without a work permit. i'm not sure about being a non-exec director and/or attending board meetings.
false negatives can occur during the incubation period, which is why quarantine exists, and part of the reason why western countries are completely failing to get the virus under control.
you are right there. they should try to find a way to rescue as much of the international tourism as possible but should prioritise the big asian markets, which are also characterised by the best risk-return balance, as well as having the most potential for future growth.
what makes you think the bars of pattaya or koh samui would be a priority for government policy making? no other nationality of tourists comes close to the contribution of the chinese to the thai economy...